Tony has been on the verge of killing chrissy a few times now or at least dropping him as a burden(The drugs-intervention)(Ade situation-when tony and ade when suspected of a relationship)(when tony told him to either be loyal to him everyday or leave tonys life) this was the final straw. Chris deserved it. He has been saved many times by just being tonys nephew and after everyting tony has done for him he is still using. He is weak and got what was coming to him.

Travis Bickle: Loneliness has followed me my whole life, everywhere. In bars, in cars, sidewalks, stores, everywhere. There's no escape. I'm God's lonely man. -Taxi Driver