Murder is the taking of innocent life by humans. We, on Earth, have no idea what the arrangements were made before we appeared, but it is logical to assume that we are on this world of our own free will. (To assume otherwise would be to rob man of that free will.) If all beings are eternal, as I believre, then their existence cannot be snuffed out by God or anyone else. You kill the body and it dies. The spirit lives on. The "soul sleeping" doctrine is very similar to that of certain evangelicals in regard to the "rapture." The former was formulated by the Adventists, which include the Seventh Day Adventists, its spin-offs and the Jehovah's Witnesses. Neither doctrine is biblical, but are manmade.
The point is, you take someone who is spiritually and morally depraved beyond your imagination (like the early Canaanites), who throw their children into furnaces and practice ritual and debase sexual practices, and you clean out a land so as not to pollute a people the Lord is attempting to raise up. That's why He destroyed the people in the flood and other places. When a people become so disgustingly profligate as to destroy the chances of succeeding generations to grow up righteous, it's not only better for them, it's better for their children and their children's children. You may kill a man with the sword, but his spirit will continue to exist in a remedial (albeit unpleasant) environment where, through the blood of Christ, he will eventually be purged of his sins. His offspring, being innocent, will also continue on as spirits and gain eternal life (also through the blood of Jesus Christ). Perhaps it is true that men will be their own tormentors in Hell, seeing and experiencing the things they did to others in life.
Whatever your views now may be...no matter what saucy repertoire you have planned with God (if there is a God), the prophets say the day will come when every knee shall bend and every tongue confess Christ. Naturally, you won’t be able to see how that will happen now, but it will happen.