Is that why Tony bagan to dial 911 and then stopped? To take Chris out of his misery? Or was it because he realized that if Chris lived and was found to have drugs in his system that he would have been busted and could have turned state's witness against Tony to make his own deal?

My friend, I think that you need to re-watch that episode.

Tony murdered Christopher. The episodes before this set the stage, and the conversatiosn with Melfi after he killed him confirmed it.