Let’s do a thought experiment!
Let us suppose that every one of the seven billion plus humans on planet earth are instantly given the individual ability to have just one other person on earth die immediately, and that there can, and never will be, any repercussions of any kind, to any person who has accepted this power and used it, neither on Earth nor in Heaven forever and ever, Amen.
How many people do you think “we” would get rid of? How many people would be still around? Would you be one of them? Are you sure?
It seems to me that even given the in-place Laws against killing, vigilante actions, and simple revenge in the heat of the moment, (not to mention, the religion based promises of severe eternal retribution for willful murder), there still seem to be too many folks out there that are just OK with other people being killed, if not by them personally then by “the good guys” (or, mere complicity with, or inaction against, or self convenient support of governmental policies: WAR.), but that's another question.
Could it be that it is really the getting away with it that is allowing this to happen? Does your God or government let you get away with it? The majority of the earth’s people say that they believe in God. Is He on the side of killers? If not, where does this all come from? The majority of people on earth obey their governments' laws. Is your government on the side of the killers? Which side, which killers? If all the governments say that they are right, and all the various “Gods” are right, who is doing all this “wrong” killing?
It probably won't keep me alive, but I've decided to not kill anybody, even given a guaranteed chance to get away with it.