Fionn,
The only difference between this man and the 9-11-01 hijackers is that they killed more people than he. Other than that, I have no answer to the problem of killing killers for killing killers for killing killers.... The disagreement can go on forever.
I will only say that I believe in the Father, Son, and Holy Ghost--and that the fundamentalists on all sides (Moslem, Christian, Jew) are destroying this world.