1. Commonly used to address another Marine and has no real negative connotations (unless spoken sarcastically, at which point the speaker is likely to spring a rather nasty leak).
2. Used to address a Marine who's name is not known (before nametapes on the utility uniform became standard).
"Hey Devildog, what unit you with?"
"Hey there devil dog, you keep shooting like that and there won't be any enemy for the rest of us."