People who think that the "no kill rule" is bad for superheroes, you really need to stop and think about the long term reprucessions killing can have. Superheroes are not soldiers and they are usually NOT hired hands. The one that are happen to be the exception and they are protected hence why they don't even need to hide their identities. In their case, killing is a last resort yet an accepted option.
However, in the case of vigilantes like Batman, Superman, Spider-Man and more, it's not just the ethics (which is valid within itself). It's the fact that they are literally not licensed to kill and when you don't have a license, killing makes you a murderer according to the law even if the victim was scum. And if you are now a murderer, the law hunts you down as a priority. Not to mention, they have a higher chance at finding you because a dead body tends to be a big breadcrumb to leave behind. And vigilantes are trying to protect their secret identity so the mercy tactic isn't just about preserving their humanity. It's about making sure the blood on their hands never gives them away. Think about it. In the AUs where these heroes kill, people find out who they are and they tend to get caught and/or lose everything.
So yes, in a sense, vigilante type supers are afraid of killing, but this fear is very much valid. They are at the end of the day "home bakers" not "pastry chefs". Technically, they are criminals from the moment they don masks and take the law into their own hands. It's a lifetime tightrope walk of breaking and bending the law while making sure you only break the ones that won't make people think you're one of the psychopaths. It's why Jim Gordon trusts Batman and why other heroes like Superman, the Flash, Spider-Man and more are beloved.
The Punisher is the exception not the example of what other vigilantes should be like. Moreover, even then, we need him to remind us what going down the dark path can do to us (not that I am counting what bad storytelling did to him, but that's another topic entirely).
The point is that the "no kill rule" isn't a stupid or counterproductive rule. It's common sense when you're not only on your own. The government is literally watching you and waiting for you to step out of line.
PS: In the case of supers like Optimus, do you have any idea what having blood on his hands would do to his image as the Prime? Look at how hated Elita Prime is because she is violent. And for the record, Optimus Prime is a hundred times more powerful than Elita Prime. If he ceased holding back, that would be disastrous.
And another thing to bring up instead of “why doesn’t this unlicensed man in a cape kill Joker?” Ask “why doesn’t the Justice system execute him?” That IS there job, it’s not Batman’s job to be judge, jury and executioner
It’s the job of the actual judges, juries and executioners
Why don’t these civilians who want Superman and Batman to kill bad guys do the job themselves if it’s so simple? Self defense laws exist, if the joker is coming at you, if you have a firearm use it, if you have pepper spray, use it, if you have a pocket knife, a bat or other sport equipment, or if your in your car, run the asshole over.
But no, most people in these just stand there and scream or run, I keep pepper spray, two knives, a crowbar and other nastiness in my bag (most of my co-workers have knives actually )that I carry into my job because I never know what is going to happen every day, customers can get nasty and if I need to defend my co-workers or myself, I’m going to do it, heck I even have an exacto knife in my apron for boxes and such at all times when I’m working!
Yet the people of Gotham never seem to have so much as pepper spray on them? You know a bunch of psychopaths invade the streets on a regular basis, why don’t YOU do something? You can kill in self defense or in defense of others, that’s Biblical! Law since the start of time, so yeah.
But yeah, it’s annoying both in universe and out of it when people ask “why doesn’t Superman/Batman kill?” And never seem to ask why the Justice system doesn’t do its job or why the civilians don’t arm themselves or force the courts hand.
