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