The reasons the Aṅgulimāla sutta (MN 86) is so appealing are many, and a few are explored below.
Late commentaries present Aṅgulimāla as a well-intentioned serial killer: he's a serial killer, yes, but out of being misled by the advice of an ignorant teacher about attaining Liberation! The early texts though do not really provide such a background, so it's possible that these late commentaries primarily attempt to solve a difficulty, by filling in the blank… And the difficulty at hand is that, in MN 86, a serial killer becomes an advanced disciple and even an arahant, in spite of a very heavy karma.
It seems to go against the very idea of kamma —if one erroneously interprets karma as a “just world” theory— except it doesn't! Kamma is not deterministic, and the very idea of Buddhism is that it's possible to free oneself from karmic bonds! So there's no need to pretend that Aṅgulimāla had wholesome intentions, to make his awakening more acceptable.
A later Mahāyāna text, the Aṅgulimālīya Sūtra, reframes this story in relation to the doctrine of “buddha-nature” (tathāgatagarbha); but it can be a controversial text as it seems to integrate Vedantic / Hindu theories (seriously at odds with selflessness, hence contradicting the four seals).
One of the core texts of Buddhism is the sāmaññaphala sutta (DN 2) on the fruits / benefits of the holy life. In particular, it asserts that a king, just like anyone else, would “pay homage to [a renunciant], rise up out of respect for him, invite him to a seat, and invite him to accept from us robes, almsfood, dwelling and medicinal requirements. [A king would] provide him righteous protection, defence, and security” —regardless of how low in society the renunciant previously was.
This sutta is one of those challenging the caste system, but is also instrumental in prescribing how lay people ‘should’ treat monastics.
The Aṅgulimāla sutta rebounds on this description, with humour. After the King described chasing the criminal Aṅgulimāla as the reason for his presence, the Buddha asks
Great king, suppose you were to see Aṅgulimāla with his hair & beard shaved off, wearing the ochre robe, having gone forth from the home life into homelessness, refraining from killing living beings, refraining from taking what is not given, refraining from telling lies, living the holy life on one meal a day, virtuous & of fine character: what would you do to him?
Naturally, the king starts with what ought to be answered. This might seem surprising: the king seems particularly ready to accept the idea of change, and to transmute a penalty of death into some respect and support!
It quickly becomes clear why. He doesn't believe the possibility even exists:
We would bow down to him, lord, or rise up to greet him, or offer him a seat, or offer him robes, almsfood, lodgings, or medicinal requisites for curing illness; or we would arrange a lawful guard, protection, & defense. But how could there be such virtue & restraint in an unvirtuous, evil character?
The Buddha doesn't even answer to such a ‘how’, most probably because of selflessness. Without ‘inherent’ tendency, time is irrelevant; it's not even a hint of one's nature. No matter how long you held onto a glass, it only takes a moment to open your hand, to ungrasp the object, to let it go and see it smash onto the floor. No matter how long one acted evilly, it only takes a moment to ungrasp the view that caused such behaviour.
Now at that time Venerable Aṅgulimāla was sitting not far from the Blessed One. So the Blessed One, pointing with his right arm, said to King Pasenadi Kosala, “That, great king, is Aṅgulimāla.” Then King Pasenadi Kosala was frightened, terrified, his hair standing on end. So the Blessed One, sensing the king’s fear & hair-raising awe, said to him, “Don’t be afraid, great king. Don’t be afraid. He poses no danger to you.”
The sutta itself thus asserted that the very idea that a serial killer could fundamentally change was hard to accept for ordinary minds… without even considering the attainment of nibbāna!
The sutta doesn't ignore the difficulty; it explicitly addresses it, head on. The sutta is both funny to read / listen to, and counter-intuitive: it is popular because it activates curiosity in a light-hearted way.
Another reason the sutta seems hard to accept is that either the Buddha is suggesting to lie (thus, to breach one of the core precepts of Buddhism!) —which is difficult to accept, even based on the intention of supporting the wellbeing for the woman and for her fetus— or rebirth is much more complicated than it seems:
— In that case, Aṅgulimāla, go to that woman and on arrival say to her ‘Sister, since I was born I do not recall intentionally killing a living being. Through this truth may there be wellbeing for you, wellbeing for your fetus.’
— But, lord, wouldn’t that be a lie for me? For I have intentionally killed many living beings.
— Then in that case, Aṅgulimāla, go to that woman and on arrival say to her, ‘Sister, since I was born in the noble birth, I do not recall intentionally killing a living being. Through this truth may there be wellbeing for you, wellbeing for your fetus.’
The sutta clearly suggests that (re)birth / death isn't necessarily, or directly, tied to our ‘physical’ birth / death.
This goes against many interpretations, from suttas where it's clear that rebirths (in these other suttas) mean over physical lifetimes / embodiements, to commentaries…
Notably, it goes against Buddhaghosa's analysis, in his Visuddhimagga, of the 12 links of Dependent Origination (or dvādasanidānāni) over 3 lives (past, present and future)… although one has to be careful here: the Visuddhimagga is popular, for sure, but it is not the sole text of Buddhaghosa and he also wrote e.g. the sammoha vinodani, a commentary to the Vibhanga of the Abhidhamma Pitaka, in which he explains Dependent Origination within the space of one mind-moment! Even the Visuddhimagga describes death as “ It has the characteristic of a fall. Its function is to disjoin. It is manifested as absence from the destiny [in which there was the rebirth].”
Birth and Death then might not refer so much to physical birth and death, but to the birth and death of our self-concept, the "emergence of the ego". If one looks at the 12 links, they primarily cite atomic phenomena (dhamma) rather than conventional phenomena (like e.g. people), so it would make sense that the birth and death apply to atomic phenomena, e.g. a self-view, rather than ‘entities’.
In a sense, of course, the sutta is consistent: if dvādasanidānāni can unfold in the duration of a single mind-moment (and in theravāda, it takes 17 mind-moments —cittakkhaṇa— for a single material-moment —rūpakkhaṇa!), then Aṅgulimāla can free himself from much unwholesome kamma in a very short time. Under the right circumstances and conditions, what usually unfolds over many lifetimes might just need many mind-moments.
This theme reappears later in the sutta, with
Bear with it! The fruit of the kamma that would have burned you in hell for many years, many hundreds of years, many thousands of years, you are now experiencing in the here & now!
Again, the sutta is popular because it activates curiosity and throws controversy up in the air: it challenges naïve, narrow, simple understandings of kamma / causality.
But then it's also popular because, by such a challenge, it provides serious hope for ordinary people: we all failed, we all acted in poor ways, we all made mistakes… This sutta makes clear that we don't need to worry too much about the ‘accounting’: all the past —good or bad— might be settled in very little time. It might not be the most pleasing experience, depending on one's past, but it's far from a condemnation to perpetuity!
Just like the sutta questioned naïve understandings of causality / kamma, it also trashes naïve understandings of the cessation of dukkha, the cessation of suffering, the end of stress…
After Aṅgulimāla has attained nibbāna, and is thus an arahant, old kamma still catches up with him: he might well be immune to suffering / stress / dukkha… but not immune to having “his head broken open and dripping with blood, his bowl broken, and his outer robe ripped to shreds” by people.
It shouldn't necessarily be a surprise, it shouldn't necessarily be seen as a contradiction: the simile of the “two arrows” clearly distinguishes ‘pain’ from ‘suffering’, the Buddha himself had to deal with chronic back pain, etc. Yet, beginners often confuse the “cessation of suffering” with ‘bliss’, or at least with the “absence of pain”; it isn't so.
Again, the sutta is popular because it brutally tackles naïve, narrow, simplistic interpretations of nirvāṇa / nibbāna.
But, ultimately, what really makes the sutta stand out might well be a stylistic feature.
The Aṅgulimāla sutta is one of the rare suttas in which a plot unfolds, things happen… It's not just the Buddha bumping into a conversation, or perceiving a misunderstanding and calling on the ignorant to dispel it… bla bla bla.
There's a rich story: cowherds, shepherds and farmers telling the Buddha not to go somewhere, only for the Buddha to go anyway… a miracle… a king play-acting as the strongman pursuing a criminal, only to later find himself face-to-face with the said person, by surprise, possibly in grave danger for not having recognised him, afraid…
There's a multi-level story, Aṅgulimāla appearing (to villagers) not only as a potential danger for the Buddha, but also appearing as the prey / target of the king… He has the body of a serial killer, he's nonetheless an arahant… Labels, labels; which one is truth?
There are multiple time-scales: when Aṅgulimāla is already an arahant, old kamma nonetheless catches up with him. He may have ungrasped wrong views very quickly, but clearly not everyone around has!
Not only the sutta keeps challenging our expectations and any naïve understanding of the Dhamma, but also it achieves such a feat through peripeteias, action, drama… a narrative, not so much a dry philosophical debate with monks seated in orderly fashion!
As such, Aṅgulimāla sutta (MN 86) might well constitute one of the top jewels of the Canon, and a great example of “expedient means” / pedagogy (which entertains in order to actually transmit deeper wisdom).