I think that Tony very cold-heartedly murdered Christopher, and did so to avoid being given up in exchange for drug charges. When Christopher admitted that he wouldn't pass a drug test, how would Tony know if he also could have been facing a possessions charge? If Chris had been holding, he could've been facing some serious time, and he might have made a deal to spill his guts in exchange for lesser charges or immunity on the drug charges.

However, I didn't think that Tony said, "I get it" at the end, I thought he said, "I did it," and that it was his way of confessing to Chris's murder. I think he was relieved to be rid of a potential albatross, but I think that he also felt guilt, especially after he saw Kelly nursing the baby. Just not sure.