Pretty sure defeating the arch mage, who is evil, is part of the College questline.
That would be a spoiler if it was correct.
Savros Aren, the Archmage, is a levelled NPC as http://www.uesp.net/wiki/Skyrim:Savos_Aren shows. He does have an impressive spell and perk list for an NPC but just how strong he is will depend on the PCs level when they first meet him.