In the immediate sense, it stops that person from bashing your head in with a mace. Assuming that you consider yourself a good person (and who doesn't?) and your enemy a bad person (a logical assumption, since he's trying to kill a good person... namely you), this could be seen as making the world a better place.Help others? How does bashing somebody's head in with a mace help others.
In the larger sense, the whole idea behind "just war" doctrine is that war can be just if the evils brought about by war are less than the evils that would occur by doing nothing. If bashing somebody's head in with a mace stops him from participating in the slaughter of a village, then it could be seen as helping others.
You might want to read the Art of War, too. Sun Tzu talks about a number of techniques to minimize killing, such as avoiding situations where your enemy is given no option but to fight to the death. He points out that if you leave open an avenue of retreat for your enemy's troops, then they will flee in disarray rather than fight to the last man. If this accomplishes your goal, it's preferable to completely surrounding your enemy and entering into a protracted battle where all their men die and your troops take more causalties than necessary.To end a war quickly, you have to kill alot of people very quickly. I cannot recall any ways before the 20th century which would support this. world war 1 and 2, yes. Perhaps the civil war, but certainly nothing after the 1700s, where a war could not end quickly anyhow.
A war can (but does not always) end quickly any time one side recognizes that the war is unwinnable. This can happen with any types of weapons in any era.