Look at it this way. Iran is essentially Islam. If you read the Koran and Hadiths, you will see that Islam is peace for Muslims, and violence for everybody else. Getting rid of Muslims might seem wrong, but it is peace for everybody else. In addition, since the people of Iran generally reject listening to talk from Christian religions, they aren't going to be saved for eternal life. This means that killing them off - since they reject Christianity - is saving them from 'sinning' more. This means that they won't have as much to pay for in Hell. So, cruel as it might sound, we are saving them from greater eternal punishment when we kill them off while they are young.