You have answered this very well in the next part of your post.
Very simply, this...
... Simply isn't fun for me.
Morality is a great thing to be in tune with. There are a lot of great questions about what makes something human. When is it morally justified to kill? Even in the case of self defense, when is the threshold of safety crossed? When do you have a moral obligation to retreat? Is it different if you present as a threat to your opponent? What constitutes threatening your opponent? These are all really important questions that I am glad people think about in real life. And if you like that in your game, by all means, have fun with it.
But I really don't want to do any of that at the game table. I don't play RPGs to have moral quandaries or discussions. That's just a level of reality I don't need in my escapism. We raid dungeons. We stop the evil that's destroying the world. We kill people and take their stuff. We don't wonder if the gods are real (because we can literally talk with them), and we don't wonder about what happens when you die (because some of us have been there), and we don't spend a lot of time discussing when it's okay to kill and when it isn't.
Every group draws a line in the sand for when they accept violence and death as an option. For some, it's practically never. For you, it's self defense. For a lot of us, we just need a quick and easy way for the GM to communicate that line to the players, and then we get on with the rest of the game.