First, or perhaps the opposite – he might be grateful that you gave him another chance.
Second, are you suggesting that this guy would live in the city of those he hates, and why would he kill the savior instead of these scoundrels who are talking?
Third, maybe he's not as stupid as you think, or the savior isn't so foolish as to abandon him to his fate after saving him!
Fourth, and to continue being an executioner with a clear conscience – no, thank you!

Except for the last point, I can't agree (although I would like to). Muslims have a completely different psychology and philosophy. It's not just a different religion – it's a different way of life (as is the case in any country). In Islam, if a warrior dies fighting against infidels, he is guaranteed to go to Paradise (or something like that. I'm not very familiar with Islam). In Orthodoxy (as in Catholicism – I don't know), it's a bit more complicated, but the essence is the same. Christians who die for their faith or for their homeland are personally welcomed into Paradise by God! So don't jump to conclusions.