We are only just finding out about Paul and so far we have seen that he is loving, caring etc etc and he does have a ruthless side to his nature with regards to business. However I like the idea that he has a dark side to his character but I don't think that he would be capable of killing anyone, only in self defense or aciidentally or if Michelle was in danger.
Maybe Ryan's father did something "sinister" to Ryan and Paul found out an argument/fight broke out and Ryan's dad was killed so Paul put him in a car to make it look like an accident.. I wouldn't mind something like this either or like yourself Paul being responsible for the crash...
I think what would follow and could be his exit would probably be rejection by his family, Liam/Michelle/Carla/Ryan, maybe even guilt, all of which would eventually break him totally, too much for him to cope with and he'd end up having a heart attack or he'll end up taking his own life, Paul's family is everything to him.