How would Harry Potter been different if it was a dark adult book?

You are giving me way to much fun with this question.
Lets see, where to start...
Alright, first, Dumbledore would have been killed shortly after Harry met him so that Harry still felt he was seperated from everyone and would be more vulnerable to the Dark. Ginny would ever exist because she's annoying. Ron would have been more leader and domineering and Hermione would be a lot closer to Harry. Sirius would have died by the dementors, leaving Harry depressed. Snape would try and help harry end they'd end up involved but in Harry's fourth year he wouldn't be able to escape from the graveyard so he becomes the slave of the whole dark side and a sign of reign and oppression to those of the light side. That means that movies 5-7 are null and void but would actually be filled with details of Harry's life as a servant and sexual slave of the dark side. I think that Hermione would have been caught and made a slave as well but Ron would have died in fighting because he's kinda stupidly brave like that. Hmm... I think that near what would be the end of the seventh book Harry will be tossed to one of the lower death eaters and then bought by Severus just because I ship that couple and then you end on a cliffy of some dramatic 'Harry is about to die' proportions....
Yup. Sounds about right. I have more if that's not enough, but I don't know if this answer box has a limit.