Is it? Zeus killed Aesclepius because he was brining people back from the dead, and thus threatening the natural order. That story was written by the Greeks more than 2 millennia ago.
Well, Zeus is not a representation of the "natural order" in the sense we mean it today. Zeus was instead a figure of patriarchal power, and he killed Aesclepius either for taking money for the service, or for causing familial/political upset among the gods (Hades, fearing that he'd get no more spirits of the dead if the practice caught on, asks Zeus to stop Aesclepius), depending on which version you read.
Neither of which is an issue of "natural order" in the modern, pseudo-druidic, sense. They had Zeus' prescribed order, which isn't the same thing, as Zeus is a pretty petulant figure with all-too-human motivations.
Plus, Aesclepius wasn't practicing what we'd call necromancy today. He was practicing outright resurrection, which is outside of necromancy's ability to create a semblance of life.
Why is magic that causes death or deals with death evil, but using a sword to cause death not evil?
To clarify - it isn't magic that "causes death" that is evil. Fireball causes death, and it isn't evil. And similarly, swords aren't evil. They are physical manifestations that just happen to cause damage to your body. You could cut yourself with a knife, or fall (probably drunkenly) into a bonfire, and get much the same effect. A fireball is just *bigger*. The high-level swordsman is just a bit better than you. But the battle is still in the physical realm. They can kill your body, but your soul or spirit lives on.
Magic that deals with death is qualitatively different. It isn't in the physical realm, and it mucks about with that soul or spirit. That, simply put, isn't fair play. Using necromancy, you don't just damage a physical shell, you touch on that more mysterious and valuable non-physical essence of the person in question. Their eternal rest is in question, and that's a big problem. You, as a mortal, don't get to muck in with that.
IMO it can be evil, but it isn't always so.
And in your game, you're welcome to run it however you like. I'm merely defending the more traditional view as valid.