Yeah, I don't get that either. For me a big part of appeal of characters like Batman, Superman, Spider-Man, ect. is that they don't just instantly kill their enemies. Not that I have a problem with heroes who kill, I'm a big James Bond fan, and NCIS: LA is one of my favorite shows and it's characters have a pretty massive body count. It's just sometimes nice to get a hero isn't so ruthless all the time.
Its not about "instantly" killing their enemies, but a decision based on the act being the only choice. In
Man of Steel, there was no way to talk Zod out of his attempt to incinerate a family in the second he was trying to. This was no Donner film, where the goofy villain could be plucked by the collar and deposited in jail like laundry delivery. Quite the opposite: Zod--a more experienced and arguably more powerful Kryptonian was not not going to be reasoned with (made clear by the plan he set in motion), which was a great mirror of real life, as anyone who had the misfortune of dealing with violent or murderous individuals knows there's going to be some who--under no circumstances--are going to listen to assumed "reason" or be talked down from committing violent acts. This is the why anyone who actually watched MoS (without their self-imposed cartoon expectations of superhero characters) understood Superman's last-second decision; he did not kill Zod because that's his habit or desire. He killed when there were no alternatives in-universe. It was a powerful, effective scene, that did not water down consequences in order to fit Superman into a selective cartoon box which would not fit with the nature of the story, with its serious subject matter.
The same applies to MCU Captain America (Rogers); despite some seeing him as the most "golly gee"/"milk drinking" type of characters, he--like Superman--knew when killing was necessary. In
The First Avenger, there was no negotiating, trapping or stopping the Red Skull. None. Millions were minutes away from death, and Rogers was dealing with a man Hell-bent on mass destruction. Rogers boarded the
Valkyrie to kill the Red Skull, not negotiate with him, or go in with some psychobabble attempting to talk him down. The audience understood that because they were mature enough not to expect cartoon solutions to grave problems. In
The Winter Soldier, when it was time to stop the Hydra-controlled carriers, Rogers did not hesitate in killing any who got in his way, with one exception--Bucky--because he knew his friend had survived, but there's no doubt he would have killed the Winter Soldier if the man behind the mask was some unnamed assassin.
