Like the Aes Sedai say: "Do what you want and pay the price."
If a character wants to torture somebody, that would result in consequences. In my opinion, torturing somebody messes with the soul; the most people cannot cope with torturing someone without very good reasons or dire consequences to their own psyche. Or they are sadists or psychopaths.
As a GM you could rule that such actions influences the PC's sanity. Same with killing helpless people who already surrendered.
Karma is just another way of presenting the bill after the party. What comes around, comes around, as they say.
If the other PCs do not react to such a behaviour, then the NPCs will. It only takes some time and they learn what twisted characters those killers and torturers are, if you ask me.
I do not know about you, but I would assume that monk orders have ethics & certain beliefs and would react pretty unamused if one of their pupils and members runs around abusing his powers and the training, giving the monk order a quite messed up reputation.
But in this case the problem is not the PC, but rather the player. If he wants to run around and mess up with the other players and the GM, disrupting the gameflow, he ... well, he disrupts the gameflow. He consciously and willingly endangers the fun of the group, including the GM. Perhaps he had a bad day or an unsolved resentment, which would explain but not excuse such behaviour. If he was just being a spoilsport and a partypooper, because he thought this would be fun, then he has a lot to learn - maybe somewhere else, where he does not disturb the group. I am sure he knows how to close the door from the outside.
*edit*
If the player did not enjoy the adventure, common courtesy dictates a respectul approach. Respectful to the GM and to the other players. He could have communicated his dissatisfaction in another way, if you ask me.