So this guy abandons Harry twice and we're supposed to believe he's a good friend? What was Rowling thinking?

Its only due to his friendship with Harry that Ron is stuck in that situation in Goblet of Fire. If he was best mates with Neville Longbottom he would never have been put into a homoerotic underwater escapade.

Harry is damaged.

>In the later years he is a prefect and on the quddich team.
Yes but he's jealous of Harry in the early years, before all that. He has his little tantrum in the 4th movie, before he becomes a prefect or starts playing quidditch.

>what happened to draco in the end, didn't he go full nazi or smth?
They don't make it 100% clear but Draco is married, has a kid, and in the epilogue Harry at least acknowledges him with a nod. So at the very least I think Draco backed off of the whole wizard nazi thing.