I have to disagree. Stevil may have had a different background but there were plenty of times along the way where he could've thought better of his actions, could've developed a conscience, and stopped short of his vilest deeds. He may have not chosen the circumstances of his upbringing but he had free will throughout. Many people come from crazy, difficult backgrounds or are raised on hate but yet find it in themselves to rise above that. Stevil didn't. He embraced it. Ultimately, the fault lies with him.
And the memories weren't implanted. This wasn't a story about mind control. Stevil was his own entity.