Re: Sopranos question. **** spoiler alert****
The point was that the entire crew was falling apart. Christopher's relapse was a sign of things never really changing. The feds were closing in, and the crew was never getting out, not necessarily because Christopher was going to flip (although that's the literal narrative), but because these guys were always gonna slip back into old habits.
And if I recall correctly, Tony discovered Christopher's relationship with the Juliana Margolis character prior to killing him. And his doing it personally was an example of his temper and flash rage decision making always going to be an issue.