@Tamerlane Look at it this way, murder is seen as an inherently evil act. Sometimes, though, killing someone can be for the betterment of everyone. An example would be murdering a serial killer. No matter how justified it is, it's still murder.
In an extreme example, consider Arthas's culling of Stratholme in Warcraft. Grain stores in the city had been deliberately infected with a disease that would kill the citizens and turn them undead. Arthas knew that if the city turned, the undead force would be so massive it would overrun the surrounding kingdom. With no way if discerning who was and who wasn't infected Arthas made the choice to kill every single person, infected or not, knowing that if even one infected person were to make it out alive it may have all been for nothing. Arthas saved Lordaeron with his actions, for a time anyways; if he hadn't have culled Stratholme, the kingdom surely would have fallen then. That, however, doesn't make his actions any less evil.
Path to hell is paved with good intentions, and all that.