I don't think Tony killing Christopher had ANYTHING to do with the car seat. I think Tony is using that excuse to make himself feel better about what he did. Chris said he wouldn't be able to pass a drug test. That cinched it right there. If Chris is arrested, he may be tempted to flip in exchange for immunity or a lesser sentence. In Tony's mind...he had no choice. With Chris fooling around with drugs and alcohol again, Tony decided to kill him.

And he's still a bastard for doing it.

I agree, SC. For the first time...I find myself detesting Tony Soprano.