Although it does sound awful, in some cultures, especially in the ancient times, it was not considered immoral to kill many people who did no one any harm. For example, early slaves were not considered human beings and it was OK for their master to kill them even for no reason. During crusades, it was fine to slaughter the entire population of different religion cities. Romans did treat conquered nations as lower humans, and killing/enslaving their population brought them fame and respect, not moral condemnation and shame.

Even today, there are extreme situations in which stabbing someone who did no one any harm could be disputable. For example, imagine this: you have the option of killing an innocent old man who did no one any harm and you know that if you do this, you will save half of the humanity, if you don't, half of the humanity will die (let's say that the innocent old man has some new, extremely contagious, lethal, and incurable virus, and there is no time to quarantine him - it's shoot now or let half of the world die). What is the moral thing to do? It is very disputable and subjective.