Indeed, Clive, and I do see your point, however, it is a person talking; in fact, it is the second sentence of what he is saying, hence the lack of "if" in the beginning. Also, the slang of "wanna" is often heard coming out of people's mouths. Are you saying, however, that due to the "wanna" (instead of want to) and there not being an "if" in the beginning, that it is no longer improper to not h
If you want character realism, I suggest this approach. Do you think the character would pause before saying 'then believe'? If so, put the comma. If not, don't.
Thanks, Clive! That's very helpful! No, I do not picture him pausing in the least. I never did, which is why I never felt there should be a comma there, so I guess I'll leave it as is. He is a fast talker, especially when he's on a roll.