I have a theory, well, I'm a few weeks behind in Ireland so developments may have happened that I haven't seen yet.
Anyway, perhaps Darryl isn't 'bad'. It seems to me that the only 'bad' ones are Ma Braxton and Heath, perhaps Darryl is the leader of the gang to keep Heath and Casey out of trouble. He knows he's much smarter than Heath, and that if Heath was the leader of the gang, they'd all be in prison by now and that Casey would be more deeply involved with them. He's the oldest of the brothers and perhaps feels responsible for them. I don't know where Da Braxton is and when he left, but it looks to me that Darryl is the one who is raising them and knowing he can't stop Heath from growing and dealing and generally getting up to divilment, at least Darryl is smart enough to outwit the cops, Heath definitely isn't.
I don't think Darryl is involved for money or profit, he's only involved to keep Heath out of prison and Casey out of the gang.