Pain is oftentimes the best prevention method.
Rex suspected that something had happened to Naela to push Kyran into this blood frenzy, he remembered back when he asked Naela, she told him that she was making quite steady progress in opening up Kyran and his iron heart.
It seemed she managed to soften his heart and made Kyran care about her.
So much that he went berserk like this.
But the problem was Kyran didn't focus his anger on a singular goal.
He was so consumed with anger he cared nothing other than killing anything around him.
Alas, the influence of the approaching Demon Moon might pose—as an influencing factor but it was still unacceptable. Rex taught Kyran a lesson, with his claws, he wanted to make Kyran remember that before anything, the one who caused the pain should be aimed first.
Failing to protect a dear one is one thing, and failing to achieve vengeance is another.