To be honest, I think that people were more alright with most marvel heroes killing than they would with anyone else, if only because of the circumstance. Cap was in a war, Iron man was fighting terrorists, black widow and hawkeye are government agents, Hulk is....well a giant green rage monster who is barely in control of himself, and Thor comes from a different world entirely. Plus, they were all fighting an alien invasion. the X-men.....Wolverine I think gets a pass because he has always been more willing to kill, the whole "I'm the best at what I do, and what I do isn't very nice" thing. Plus, alot of the time these guys are put in positions where they don't have much choice but to kill in the movies. Another important detail is that these guys would be more willing to kill even in the comics (although I don know they go back and forthe on whether or cap killed anyone in the war or if the Hulk killed anyone in his rampages). Plus, alot of the time these guys are put in positions where they don't have much choice but to kill in the movies
Now compare that to how people reacted to superman killing Zod. People flipped because that is just not superman. Superman (alongside batman), is pretty much the embodiment of "though shalt not kill" in all other versions, only ever resorting to killing in the most extreme circumstances. Spidey is also the marvel character who more or less embodies that idea (though this does depend on if its a day where the writers say cap did or din't kill in the war).
However, I think people losing their shit over supes killing zod proves that even if you make a movie about a hero who doesn't follow the golden rule, It can still be successful.
However, in looking through the comments here, it astounds me to see people saying that the heroes should be the ones to kill people, instead of just letting them get the death penalty. I've never really gotten why people think its so much better for one man to decide whether someone should live or die, instead of whatever the law says should happen them. It almost seems like people just want to take the responsibility of taking the life o a criminal that deserves to die from the system that we use, and give it to someone who is outside the system simply because we don't like how the system works.
a super hero can save the world, but they can't fix it. that is ultimately left up to society, which seems ready to give that responsibility to a super hero at every opportunity.