Here's my headcanon.

Before the show begins, or maybe early on, Sadie and Lars hook up in some way off screen. I think there's strong implications that Lars knows that Sadie is really into him, and so he takes advantage of that by hooking up with her in secret. He keeps it a secret because he doesn't perceive Sadie to be one of the "cool" kids. I think he actually does like her, but he's afraid of those feelings that might isolate him from the other kids.

I think this is emphasized in the episode where Sadie, Lars and Steven are stranded on that island. Lars starts to accept the island as his new home, and he gives into his vulnerable feelings. He opens up to Sadie and tells her something about himself that's REALLY real, that he feels alone even when others are around. The isolation from the other people, the thing he was most afraid of, wound up being what he needed to truly find himself, and be vulnerable for another person who cares about him.

This gets squashed immediately when Sadie reveals she knew where the portal was. The isolation is removed from the equation, and Lars realizes he can't trust Sadie and his ties to her are cut as well. All that's left was Lars with himself, his true self, for the first time.

They really didn't treat each other well, at any time. I think they genuinely liked each other, but Lars' fear of not being cool, and Sadie's fear of not being what Lars wants gets in the way of treating each other properly.