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 … … … …