3. The Villain Must Destroy Himself
It's always been a fundamental rule that superheroes can't directly kill super-villains. Though that kind of make sense, it also kind of doesn't make sense
at all. If they just did away with the super-villains when they had the chance, thousands of people would probably get to live in future scenarios. Oh, well, that's probably for another discussion. What we're talking about here, though, is the moment when a superhero has the chance to kill a super-villain, stares poignantly at them for a moment, and says, "No. You're not worth it." Then said superhero turn away, only to - surprise! - find themselves being attacked from the rear. Thus, in the process of their "deceitful attack," the super-villain accidentally gets themselves killed. You can check out a prime example of this kind of situation at the end of the first
Spider-Man movie, when the Green Goblin tries to stab Peter with his hover board but ends up impaling himself in the crotch. Though this all stems from the fact that a superhero shouldn't really kill a super-villain, we say, um, maybe sometimes it can be justified for them to just go ahead and do it anyway?