Superheroes and supervillains fight because that's what they were drawn to do, but when two heroes go at it there's usually some underlying reason involving truth, justice and the goody two-shoes way.
The superhero flick Batman V Superman has brought up the sore subject of superhero-on-superhero violence, which will continue when Cap and Iron Man go head-to-head in Captain America: Civil War.
But believe it or not both Batman and Superman have been involved in far more brutal fights, with Bane and Doomsday respectively, which makes sense since heroes show restraint when fighting their fellow do-gooders.
Bane broke Batman's spine in the Knightfall story arc, which led to a lengthy recovery and Azrael taking over for Bruce as the Batman Family rallied behind their fallen leader.
Superman was "killed" by the mighty alien Doomsday in The Death Of Superman story arc, which kept Supes out of his spandex supersuit for three long years and led to a reboot of the beloved character.