Saying its only the murdering where you draw the line, and not the torturing,vigilantism and the assault, and then saying they are like citizens, seems like a contradiction.
The point of they are citizens is that they are not deputies or officers of the government of their country. If that were the case, they are under the jurisdiction of the laws of their country, and if that country allows heroes to murder, then so be it. But in our discussion, heroes are merely citizens.
Murder ends life. None of that other stuff necessarily ends life. That's where the line is drawn. Who gets to judge when life should be taken and when it shouldn't?
Technically, this is a legal discussion, because obviously it's all illegal, but in many ways it's really a moral one. Should a hero be morally allowed to take a life? I think taking a life is where the line gets drawn. If he beats up the criminal, but the criminal lives, that's illegal as well, but the criminal isn't dead. The criminal is alive and still able to live out the rest of his/her life, maybe even get rehabilitated and become a great member of society. But once he/she is killed by the hero, the hero has ended that possibility for that person.
If we are saying that in all circumstances the hero is a law unto himself, that's where things get awfully dicey. How do we know the hero is using good judgment in deciding this life? What if the hero is having a bad day and just says "Screw it, I don't have time to negotiate a surrender, I don't feel like fighting this guy right now, I can just off him?"
So legally, the answer is no.
Morally, the answer is no.
No murdering for heroes.