@nevity There's a lot about him that's suspicious, he always wears the same plastered on smile and it seems like all he does is for someone else's sake, he kinda reminds me of Nadeko or Hanekawa from the Monogatari series if you're familiar with it.
He's a very obvious foil to Leslie, both have experience with abusive parents, Leslie has dark magic powers, Konrad has holy powers (Also worth noting is that holy powers and magic don't mix) Leslie sought to escape the abuse but Konrad simply endured, Leslie likes sweet things Konrad doesn't, etc etc. By the way this is unrelated but Queen Dowager Medea is also a foil to Leslie with their similar backgrounds.
Most of all though, it's that in chapter 34 he asked her to do him a favor sometime in the future without saying what it was. Never trust someone who tries to make you owe them.
I think that at some point he's going to try to sacrifice Leslie to somehow help his little brother. I don't have any actual proof that this is what he has in mind but the family records that Leslie used to discover the history of sacrifices was stored at the church so it's not unreasonable for him to know about it. I think he would do absolutely anything to protect his younger brother, and that makes him very dangerous.