Just when you thought you know someone well, his true colors show and the friend you thought you have turned out to be a wolf in sheep's clothing, a two-faced snake...
You accepted somebody for what he is, with all his shortcomings and weaknesses, into your fold. He gained your trust, shared secrets; you helped him through his difficult times...only to have that somebody slowly undermine the friendship you painstakingly built over the years, with his arrogant act of self-gratification just because he needed something to distract him from his pathetic life. Too lazy to even look beyond the group and instead turned on the persons who considered him a friend.
Like a disease, he slowly festers and eats away at the very foundation of your friendship; he charms, jokes, and laughs his way as he alienates the friends from each other, and takes advantage of somebody's emotional vulnerability and inexperience.
What used to be an open, cheery, fun group now harbors distrust, dishonesty, secrecy, pretensions, and his favorite -- selfishness. He seems to be enjoying the discord, after all, as long as HIS needs are met, to hell with others, right? He must be grinning smugly, now that he got what he wanted.
You wish you could take back all the time and effort you invested on that person. What a waste, he does not deserve any of it. His singular act of egotism effectively destroyed friendship...