Chris would have been dead long before he got made if not for Tony.He was a loose cannon,a junkie,and a hothead. Once he did get made,Tony continued to overlook a lot of things that would have resulted in a death sentence for any other soldier.

The family connection was all that kept Chris around. Remember early on when Tony basically told Chris that he was going to isolate himself from the Family business more and more,and that Chris would be the one passing along Tony's orders,with the eventual goal of Chris taking the reins?

For a relative, Tony could overlook just about anything except the drug addiction,which in the end is what got Chris killed.Tony finally realized that Chris would never get clean.
Tony knew that not only would Chris's work suffer,but he could still get popped on a drug charge and flip. The last straw was the look on Tony's face when he saw the baby carrier in the back seat after the wreck.

Chris finally went too far,even by Tony's standards.