I'm going to work under the assumptions that the individuals are truly peaceful by nature and that this is strongly related to the mental abilities you cited - that is, if just about everyone is low level empathic, hurting other people would not be a common desire.

If this is the case, a fitting punishment for the plotter could be enhancing their receptive empathy and forcing them to serve the people currently mourning the victim for a certain term. They get a unique punishment in experiencing the pain they have caused and it 'fixes' what people of that society would see as the most common cause of violent crime.

For the outsider, a more intense version of the same as provided by projecting empaths, plus return to his own land to face whatever form of punishment they use.