Giant mythical time-breaking stompyrobots are never cliche.
They banned it becuase the necromancers were defiling corpses and kidnapped and killed people in order to extend the own life and power. Not evil in your eyes?
Incorrect. The "defiling of corpses" is not a sufficient enough reason to label them "evil." Modern day morticians or coroners "defile corpses" too. The "defiling of the corpse" is not illegal. Using a corpse without permission is, but there are plenty of ways to attain bodies legally. Either through volunteers (think of donating your body to science) or through the corpses of criminals, bodies are attained, and it is perfectly legal to study them and use them. After all, a body is just that: a body. The soul is long gone from it and is chilling in the dreamsleeve long before the body ever gets used by a necromancer.
Further, the activity did not result in the banning. The banning resulted in the activity. In Oblivion, the Mages Guild overstepped their bounds in banning necromancy, and they took the first hostile swipe. Imagine if you're studying something that's completely legal, something that you're
not using to cause mass havoc and despair on the world, and all of a sudden, some guy on a moral high-horse tells you your studies are inherently evil and therefore you must be removed forcibly from the guild. Read http://www.imperial-library.info/obbooks/black_arts_on_trial.shtml. Note that, at the end, Uliceta was discovered to be a practicing necromancer, and Traven makes the decision to send the Mages Guild's own private troops after her to arrest her. First, they have no right under Imperial law to arrest her. But second and most important, remembering that necromancy is not inherently evil, this is just as bad as someone sending out the secret police on someone because of their views. There is no evidence to suggest that Uliceta was using necromancy to damage lives or property, yet Traven automatically entwines "necromancer" with "evil" and acts accordingly, against the laws he's bound to. Which is the moral side, here? Finally, the book notes that she "makes good her escape," as though that's some sort of affirmation of the wrongdoing. Why shouldn't she have fled? She was about to get arrested by those who didn't have the authority, and potentially subjected to who-knows-what kind of punishment for simply practicing an art. Is it really not understandable, then, why all these necromancers leave and are pretty irritated and scared that their guild is dong some majorly judgmental and rash things? So when the KoW and his band come along and offer them a resistance flag, a lot of them probably jump on the bandwagon. Not to say that all of them do; that's just oblivion's failure to display it. Saying that all necromancers are evil because they're all hostile on sight is like saying that all wildlife is rabid and evil because all wildlife is hostile on sight.
And finally, the suggestion that the kidnapping and killing is being done to extend internal power and life. That's also incorrect. While practitioners of necromancy are more likely than most to find ways around Arkay's laws, it does not require them to kill to attain it. And as for the power assessment, while they might be killing to assert power for their group, they aren't killing in the fantasy sense of boosting their own individual power levels. Or if they are, it's not like they're any different from the player in that regard. The player kills a whole swath of things to boost their own power level. Pot calling kettle black?