Negan doesn't sleep with Alpha in the comics. The comic plot doesn't go in that direction. Their characters have different arcs and interactions that don't involve that kind of relationship.
Well, Negan first appears in the series as a major antagonist. He has a very distinct personality. He's confident, often to the point of being cocky. His group, the Saviors, would extort other communities for resources. He uses fear as a tool to control those around him. His actions have a huge impact on the other characters in the show, like Rick and his group.
Well, it depends on your perspective. But generally, Negan is considered bad in the comic because of his cruel and violent actions towards the main characters.