I don’t think anybody deserves to play judge, jury, and executioner just because they’re anonymous and have powers/gadgets/skills/whatever. Like, what Batman does is ethically sketchy and completely illegal, but ultimately, what he’s actually doing is committing very selective crimes targeted to leave other criminals vulnerable to arrest and prosecution. I can accept that in a world of costumed, superpowered maniacs, that’s a necessary and productive course of action to keep people safe. Killing criminals, though, has this kind of authoritarian mindset of “I know better than anyone else how to deal with problems.” Even monsters like the Joker have some rights. And in fact, he’d probably have been executed several times over if he were real, thus taking the decision out of Batman’s hands anyway.
I’m actually more OK with heroes killing if the enemies are not “normal” criminals or if there’s no other viable option. Alien invaders, for example, are more like wartime enemies than crooks. Of course, for Batman specifically, the no-kill rule is central to his characterization and has a more personal motivation, so I don’t think Batman or any Bat-Family member in good standing should kill under any circumstances, but if, say, the Flash killed a major-league baddie in a moment of desperation, I wouldn’t necessarily object if it were well-handled. Of course, that’s exactly what happened with Wonder Woman and now DC seems intermittently tempted to characterize her as this ruthless homicidal maniac, so maybe it’s a bad idea from a perspective of bad-writer-proofing.