And didn't the old kings die not so long ago? The only brainwashed soldiers would be children, teenagers, or fairly young adults. All the adults will remember what it was like to have a sane king. If you look at history, psychotic and/or murderous monarchs don't last very long even in their own kingdoms. Especially if you roll up into another sovereign's lands and start killing folks for no good reason.

Everything in moderation. Even murder, lol.