Why?

My guess is that he found out he was banging his woman.

Because he killed that other gangster boss. By killing the leaders killer he can claim he had nothing to do with it (which he kind of didn't).

A difficult one, this, but I am going with because he slept with his girl friend. Remember how much and obsessively he loves her.

Big brother was obviously treating the other boss like a superior to himself. The other one could humiliate him, ruin his business, but he would still look up to him and feel inferior despite all the hard work he put in making his way through to the top. He was obviously furious every time his guys were mentioning attacking or getting rid of the other boss (remember when they had a fight with the other boss'es men?), so he was as I imagine quite unhappy about the main character kiling him, despite the fact that he profited out of it greatly. He would not be able to go on without this killing, yet he did not want the other guy dead.