Haha. And the dude says "wait till you join the mob blah blah blah...." Like OP who happened to know a murderer is now garaunteed to join a gang who participates in murders...
I assumed he was talking about a mafia, that is to say, an organized crime syndicate along the lines of La Cosa Nostra, the Yakuza, etc. Not a gang that happens to have the word 'mafia' in their name.
Who knows why he killed the person? Maybe it was totally unneccesary and he is nuts or maybe had some serious shit with the person...
Well this is my guess as to what happened. I know the kid was pushing A LOT of weight. He was known for serving E. So i just think someone OD'd or something off some rolls they bought off of him. That is just what I am guessing. I just can't see this kid killing anyone.