Teensy spoilers for Heroes:
Generally I'm in favor of prison rather than execution. But if the person is announcing their intention to keep doing bad things once they're free, and the odds are highly likely any effort to stop that person from more killing will result in people dying anyway, and if there is no way to ensure the person doesn't get away, then yes, execution is okay.
Normal people don't have super-powers, or villainous organizations they are in charge of, so I almost never approve of killing a normal person as 'justice.' But if you can teleport or turn invisible, you ain't gonna stay captured long; the goal of rehabilitory incarceration will fail outright, so we have to look for another solution. "Super prisons" seldom work, because they necessarily require isolating the inmates and minimizing prisoner-guard interaction, which just makes the super-prisoners more resentful should they get free.
I view V's actions as firmly Chaotic, most-likely Good. Screw the law; just make sure the bad guy can't harm anyone else. (Alternately, as Chaotic Neutral: screw the law; just make sure the bad guy doesn't get in the way of saving the day.)