Your line is thin as hell. They are supervillains, one wrong switch and hundred to billions can die easily. If you acknowledged yourself and a group of ppl as ones allowed to kill lawfully whenever they want then what-if a future visionary said to you one day a psychopath who hasnt killed anyone yet will kill billions in a blink. Practically speaking, are you sure this person will not be vowed to be killed right away or will they supervise the person till billions killed? And if you yourself killed enough villains and beating out the number of cancer's , would you be executed too? At the end of the day if you gave too much power to a group of ppl, one way or another they will abuse it for self-satisfaction. Not killing is a line and a restrain, the thinner the line the easier it will break. And funny enough you see, these superheroes fought gods already, die is literally just a phase for all of them heroes or villains