An overwhelming majority of metal is super positive. Even songs like Iron Maiden's 'Fear of the Dark' doesn't sound negative although it's literally about fear. Even in extreme metal, say Necrophagist's songs like 'Multilate the Stillborn', it's all goofy and energetic and tongue-in-cheek. I've heard a lot of metal but can't think of any that is negative in a serious way. Perhaps melancholy if you consider sadder stuff by Opeth or something.