He find out that his wife is a vampire and tells you the story of how his wife hated Alva but then suddenly started hanging around with her before she vanished.

Thonnir gets defensive about Alva when you talk to him about it.

He says things like "I refuse to believe Alva had anything to do with this" and "Laelette probably met her fate in the marsh" and "There's no way you can prove it to the Jarl"

Seems pretty unusual considering that he has no relationship with Alva at all.

You think he would be figuring it out for himself and wanting Alva hung or something but he acts the opposite.

Do you think Thonnir was secretly having relations with Alva?

That could explain why he gets so defensive about her and she isn't even his wife.

I've always thought she was starting to control him and make him into another thrall, but hadn't quite finished yet.

Maybe he's in denial?