Red Mist surviving, becoming the super-villain was always planned, but the idea was that he was going to be a more minor character in the first film. Then we saw what Christopher Mintz-Plasse was capable of! Also, I really like the idea of somebody who people were relaxed watching on screen doing horrible things… the second movie, it’s going to get very, very dark what goes on with him…So the idea of McLovin’ and the fun Red Mist doing something horrible is genuinely quite disturbing when you see it happen. We couldn’t have got away with that with another actor. The minute we saw his performance, we were looking at each other and realised how good he was and what we could do with him in the future…
Scriptwriter Mark Millar
Red Mist going rogue would kind of make sense indeed since he's the son of the first movie's villain Frank d'Amico.