What is the motivation for the list of 5 deadly sins?

I'm particularly interested in if there is any evidence in the sutras (or commentaries or anything old) that explains this peculiar list of transgressions (the Ānantarika-karma): 1. patricide 1. matricide 1. killing an arhat 1. injuring a Buddha 1. creating schism in the saṅgha. 1 & 2 & 3 are already against the first precept. 3 & 4 are hard to do even if you wanted to for lack of suitable arahats and Buddhas to kill or injure. I've come to suspect that this is just a formula for expressing that schism is *really* bad. But this formula appears in many places, often without much other reference to schism. (for example ordination,