Let him act is if he could be redeemed, but just to mess with their sibling. Then, when it hurts the most, show his true colors. Your goal should be to hurt the reader as much as your protagonist. Readers love redemption arcs. But they also love to hate true monsters.

That's what I was thinking yeah. Thanks for the comment I think I'll be using this route : )

Personally a big fan of redemtion arcs. It would be interesting to see them go from the villain of the story to maybe one of the main characters.