If doing something morally good has a morally bad side-effect, it is ethically to do it, providing the bad side-effect was not intended. If that person is willingly to sacrifice themselves and to save many people, that should be enough. Technically it is not murder. Killing all of them, and not saving at least one is not ethical. If you have a chance to save a person or people, regardless on a situation, you should take it. It is not a heroic action but it is a good morality action
Clearly if killing one person will save many then that person has to have done something to deserve death. Looking back in time we can see many instances where killing one or two people could have saved millions of lives. If Hitler had died as a child due to some sort of accident look at how many lives would have been spared. You have to look at the entire picture. Not just the pieces.
Say you had one rescue helicopter, a pilot, and only enough time to reach one out of two groups of survivors. Group A is a group of 6 young people. Group B contains 2 young people. "Killing" (killing=not rescuing) group B in order to save group A would probably be a justified decision, seeing as you'd be able to rescue more people.
When you justify killing one person to save many others, it creates a slippery slope where you start defining how many people are worth saving, or "if" people could possibly killed, then you could kill someone to prevent those deaths, too. These cases lead to very unethical practices that justify murder.
A lot of people define being a hero in their own ways; personally, I do not believe in vengeance or murder in general. I don't think anyone should ever decide to resort to murder to try to save anyone from anything. There are other ways to handle this type of serious situtation.