If it is an assassin, then:
1) Who is his employer?
2) The answer is most likely New York. But then:
3) Phil's dead. Butchie made the truce with Tony.
4) Why would Butchie kill Tony? It doesn't make sense; without the direction of Phil (or the direction of Tony), the two families wouldn't have much business to attend to.
5) Is it possible that the Members Only guy was employed by Phil, and when Phil was killed, word couldn't get to Members Only? It's possible, but unlikely.
I'm not sure Tony was in fact killed in the end, although it gives you the impression he was. I personally think it's 50/50.
But to add a valid argument to a possible reason for it: you can't just get away with killing a "boss", especially one of the major New York families. Not even if you're a boss from New Jersey.
It doesn't have to be Butchie who gave the order. I agree that it is unlikely the order came from. But it is likely it came from the other families on the Commission. What signal would it give to others that you can get away with killing a boss? From their perspective, they had to make an example and Tony was a rotten apple that had to go. The best solution would have been to make a clean sweep after all the chaos, which Tony was partly responsible for.
And it's also plausible the guy could have been hired by Phill when he was still alive. Something equivalent also happens in Road to Perdition.
