The Bible does NOT say 'Thou shall not kill' ... the Bible says - Thou shall not murder.
There IS a difference and for people unable or unwilling to see the difference, it is a good thing there are people who are willing to, regardless of their worth, protect them.
There are of course restrictions. You can tighten them up, like the laws in Israel, restrict what the military can and cannot do when executing a strategic removal of a terrorist / killer. The law is - the terrorist must be en route / in-process, or getting ready to go out and kill innocents. At that time, it is legal to remove said killer.
Likewise - when a group, a person, a government, state they will kill children or capture and train them or ... it is moral and legal to protect them - including killing anyone who tries to harm the innocents.
It would be immoral to do anything less.
It wasn't so much a bolt-out-of-the-blue, as a baby that turned biker Sam Childers to God.
Against long odds, his ex-stripper girlfriend finally got pregnant, and that was pretty much it.
He gave up the outlaw lifestyle, though not the guns.
He quit peddling the dope that was killing junkies, and took his newly-reformed, God-fearing self to Sudan and commenced killing rebels so he could free child soldiers.
At least, that's his story.
It was enough to persuade Hollywood to make a soon-to-be-released film with actor Gerard Butler playing the man himself.