I was watching some battle scenes in the first Avengers movie and the Age of Ultron, and I guess it makes sense that the Avengers used lethal force on the Chitauri because they weren't human, even though I think they were being mostly knocked out besides some warships and that huge snake flying dragon machine thing.

Then, I watched the AoU opening battle and some of the stuff didn't add up. Because from what I've seen, most of the Avengers use non-lethal methods to take out their enemies (i.e. Stark used mini bullets on hydra agents in non-lethal areas because they were still breathing after they got hit), but then thee was a scene of Black Widow shooting her guns at an enemy agent so I was a little confused about that. I know that hollywood doesn't really care about the superhero code of not killing their enemies, but it was just something that bugged my mind.

I mean, in the comics...during the Infinity event, I think that was an actual war where killing was justified where they had to stop the builders, (If I remember correctly, they were just mostly non-sentient machines?) and I vividly remember in the Avengers Assemble tie-in book where one of the Accusers smashes the head of one of the members of the Black Order.

So, the question that I'm really trying to ask is: How far does the "No Killing" Code go? Do they not take human lives or any other sentient-beings unless it's an absolute necessary? Because in my eyes, if the Avengers kill their enemies every time, that makes them no different than a military force with the power to execute anybody at will when they should represent something BEYOND that.