Supervillains fight superheroes - it's the bread and butter of Marvel, DC, and several other comic book publishers' universes. However, it's equally fair to say that some of the most interesting confrontations in the superhero genre haven't involved heroes at all, but actually a villain pitted against another of their fellow rogues.
A lot of the time these disagreements emerge from fragile alliances breaking down (surprise surprise, supervillains aren't usually ones for cooperation). Other times, there's just a case of bad blood existing between two characters already. There are even occasions where a villain turns on their allies because they've had a change of heart, and have been unable to continue with a given course of action.
The fallout of these confrontations tends to generally be insignificant, but every now and then one will take things to a whole other level, and it'll result in a massive death.
These deaths can send shockwaves through a given hero's rogues gallery, but especially for the heroes themselves too. They may fight these despicable evildoers regularly, but they have a relationship with them at the end of the day, and stand to be just as affected as anyone else as a consequence.
It's a fascinating trope regardless, and one DC and Marvel have deployed with varying levels of effectiveness over the decades...
10. Ruin Kills Mister Mxyzptlk - Rack & Ruin
It might be unfair to label Mister Mxyzptlk as a straight up villain, given he's more a mischievous imp in the vein of a mythological trickster god, but it's equally fair to say that he's caused a lot of grief for Superman over the years, and has skirted into supervillain territory more than once or twice.
The character's main goal has always been to torment and test the Man of Steel, but Kal-El has proven more than capable of outwitting Mxy and banishing him back to the fifth dimension by getting him to say his own name backwards. Over time, the pair even came to respect one another, becoming friends, and during the events of the 2006 storyline, Rack & Ruin, an amnesia-afflicted Mxyzptlk was cast in unquestionably sympathetic terms as he struggled to get back home.
But a villain he once was, and during the Greg Rucka, Nunzio DeFilippis, Christina Weir and Renato Guedes comic, Mxy and Supes' friendship came to a tragic end.
During a fight with Ruin (a new villain on the block who was later revealed to be Emil Hamilton), everyone's favourite fifth dimensional imp is impaled by a kryptonite spear meant for Superman, and effectively dies saving the Man of Steel's life. Mxy tries to save himself by saying his own name backwards, before vanishing, and does later return in Countdown to Infinite Crisis, but there's no getting away from just how tragic a sacrifice his was.