Pretty sure it's mostly a DC Comics thing anyway.
I mean, even in the Marvel movies alone, there doesn't seem to be any major objections to killing on screen.
Think of how many people Iron Man killed during his missions in the middle east in the first movie alone.
Cap served in WW2, killed a bunch of Nazi's.
Thor ain't above laying the smack-down if needs be.
Now, the comics might be different and I'm sure they had to deal with that trope at some point, but I think about the DC side of things because they were the ones who made such a big deal out of it in all of it's mediums.
Hell, the entire plot of Injustice: Gods Among Us happened because of not killing the Joker or not giving him a death sentance and, later, when Good!Lex refused to use the suit of armor he designed for the sole intent of killing Superman because it was "Too Cruel".
Doctor Doom is the sweet spot IMO, thanks to his Doombots, he can be killed over and over and over and over again without really decaying the character.
Not saying that every villain needs to be able to respawn but after so many years, I imagine that some writer out there has to be able to come up with something to get around that rule on the DC side of things.