Kylo still has a grudge against Chewie for blasting him. Yeah, it was warranted because he had just killed his best friend in the galaxy, but Kylo probably doesn’t see it that way. In his twisted thinking, he probably believes that Chewie has abandoned him like everyone else and hates his Wookie guts for it.
This could have been expanded on with the deleted scene involving Chewie’s torture. IMO, what would have been a perfect reveal in that scene would be if it’s shown that Chewbacca was aiming at his head for the killing blow after he killed Han but changed his mind at the last second. That would have been a stepping stone toward Ben’s forgiveness and his ultimate redemption.