I don't believe that you have any evidence that "they had no reason not to kill". They may -- and probably did -- have lots of reasons why they shouldn't kill, but those reasons were overruled by the reason they had to kill. Just as a person driving at 120mph down a country lane knows lots of reasons why they shouldn't do it, but their reason for doing it (to avoid being late, or even "for the thrill") is more important to them. If someone is really both capable of murder and knows no reason at all why they should not do it (I'm not convinced that the state is even possible, that it it is possible to get to a state where you know how to use a gun and have never been told that killing someone might get you killed or locked up, but it may be) then that person is not responsible for their actions because they are insane, or they have been brought up as a "killing machine".

And sorry, your statement that "Murder is never justified. Not even in those rare cases where it becomes necessary" (and you admit that there are some cases where even you would find it necessary) just does not make sense at all. If it is necessary then it is justified. I will happily support and respect a person who says (as you did up to that point) that murder is wrong absolutely, and sticks to it absolutely, but to say that it is both necessary and that it can't be justified won't wash.