I perhaps worded that poorly. He seeks his father's approval so much that he questions his Task very little. He really has yet to have his morals tested. He is not an aberration, but he draws power from the same place as them. And while I don't want to reveal everything right away, Luka was mostly tricked into making the Star Pact; "harboring evil" as you say was not his conscious choice. Saying that his powers are inherently evil is particularly preposterous. How more evil are any of his spells (I implore you to look them over) than a wizard that kills his foes by melting their flesh with arcane fire? Or a warrior that caves in the skulls of his foes with a hammer?
I insist that alignment is determined by action. What has Luka done so far to make him Evil?