Tony murdered Christopher because he was a liability, because Tony was damn tired of trying to straighten him out, because Christopher was a danger to his kid: all those reasons. Yes, Tony didn't call 911 because he realized those things and this was an opportunity to get rid of him. He was relieved.

As far as AJ goes, didn't he throw the guy's bicycle across the street? I think he's hugely conflicted, but his actions are taking him down the wrong path.