Well technically it's Mista panic firing on someone who might be Diavolo but Mista does 100% drink his respect women juice. As should we all.

From the look on his face and the fact that he doesn't even try to be sure it's him or warn others, I'm pretty sure he knew that wasn't him and just wanted to waste the guy. The fact that he realizes or not that this was the body of another unrelated dude is more debatable however