I think for Tony,the final straw was seeing that the tree branch destroyed the infant seat in the car.
Along with that,and realizing that Chris could have killed him in the crash,Tony finally saw that Chris would forever be a junkie.
I agree that Chris could have easily ratted Tony out if he had gotten hit with a narcotics rap,or the killing of the Cleaver writer,or a number of other charges.
No way could Chris take a 30 or 40 year to life serntence'

As far as Ralphie I wouldn't consider that a killing in the accepted sense. Tony confronted him about Pie Oh My,lost his temper and beat Ralph to death.
I don't think his intention in going to Ralph's was to kill him. Thing just got really out of hand fast.