Reiko usually kept his emotions in check (Sometimes by you) but he never wanted to brutalize someone so badly that’s not Shao, Shang Tsung or whoever before—He didn’t like how the new soldier looking at you, his fiancé/fiancée with puppy love eyes.

It was a new recruit in the Imperial Army and he probably didn’t know that the prince/princess is with Reiko, the Lieutenant which was understandable as their relationship is secret to the public but Reiko’s point still stands.