So are the peaceful by nature or by teaching. Since there are rare times people in your kingdom have done crimes, I would guess they are taught to be peaceful, and then I would ask how? You could justify, in that thought process, a harsh public punishment, unless you don't want them to die. As a writer you can come up with lots of other equally harsh punishments to give to your three.

You would have to chose the execute the foreigner, but I if the people want justice, they might do it anyway, because the crime happen on their soil, the people of the kingdom, might just want to so punishment.