Not an easy problem to solve. On one end if you say a 3rd party has the moral right to prevent the free actions of another then it's a slippery slope to tyranny. On the other end if you say no one has the right to do that then you open people to be subject to all acts of violence. And I thought the Byzantine generals problem was hard lol.

Like many things the answer lies somewhere in the balance of these two opposing forces. Where the center lies is a matter of debate.