" Hey, we dont like you and are going to end your way of life. But dont worry because we will welcome all your people into our group so they can take their time to plan and infiltrate our entire system. So one day, when they decide we are weak, they can take us out from the inside."
Dont trust the enemy, and while it might seem cruel to kill all of them, you leave one alive, and that is one who can poison the rest. Save the right to live for those who want what is good for all and eliminate those only out for themselves.
Ideal when those who are out for themselves are A: eliminated or B: are too scared to act purely for themselves. We can actually build a society that puts the people first and make it a crime that demands exicution to put yourself over the good of the whole.
Another ramble I might be right. I might be wrong, who is to say.