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):


patricide
matricide
killing an arhat
injuring a Buddha
creating schism in the saṅgha.


1 &amp; 2 &amp; 3 are already against the first precept. 3 &amp; 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,
It is said that our current situation is the result of our karma collected over countless of lifetimes. That means that many of old-forgotten deeds sowed the seeds which ripened only after many lifetimes.

The 5 deadly sins are listed separately to highlight the fact the the results of those action will be immediate - killing a parent will definitely prevent one from reaching an arhat state in this life and the person will surely be reborn in a lower realm straight after death and will spend a long time over there. (see for example third  and fourth reference).

It has been already stated in one question that in order to attain enlightenment one does not need to purify all the bad Karma. Indeed, one of the greatest Yogis of all time, Milarepa, before finding his spiritual teacher killed 35 people and some sources even claim that all of them were his relatives. Regardless of those deeds, with the help of his guru he achieved full enlightenment in the same lifetime. If he had killed his parent though, this would have never taken place.