In Elite Dangerous Odyssey, there are mission wherecyou have to kill people. Sometime they are high bounty criminals. Other times, the corporates want you to assassinate people like unarmed scientists and engineers. Not my cup of tea.
In Skyrim, there's a quest to kill a fluffy harmless Dragon. I didn't do it.
In Fallout New Vegas, there's a whole town with Ghouls in there in NCR (military like faction) clothing. Turns out... That the opposition, who are based on Greek Legionnaires, had released radioactive substance into the town which had burned the skin of everyone and drive them mad, a. K. A. Ghouls.
I felt really bad to kill them, but it was mercy killing. Still felt bad.
Also in the same game, at one point I had to kill a slave mother and daughter because their fate would've been far more worse than what the Legion would do to them. Basically they were for breeding. You can either buy and free them, or persuade them that the girl is sick in the private areas so they let them free. But I didn't have money first time I was there, nor did I have adequate speech skill.
In Return to Castle Wolfenstein, if you save a very large non-patternic integer (1759437) and then load it, it would take a very long time to load, which in turn makes enemy encounters funny because AI will die. You will see the enemy's hidden loadout, enemies will fall asleep on the ground, will look around but don't shoot, etc.
You can kill them and then save. However, events won't happen. For example, if a boos should appear, if won't until you quicksave, exit to main menu, and then load the quicksave.