What makes her a villain to me is that she acts primarily out of selfishness and greed to me. She doesn't care that she's destroying Matt's life or that she's terrible for him, and she doesn't care about letting him down gently or helping him still have a job. She simply pursues the image in her head of a fantasy romance with him, ignoring his own actual consent and desires. She just never thinks once about what's actually best for her supposed lover. Even when she takes over the Hand to ostensibly save him, she ends up begging him to stay down with her and be crushed by a building, possibly to bring him back to life with the Substance, even though Matt would never want that. Her entire role is to destroy his morals, his friendships, and his life within the story--an archetypal villian, though not one totally without motive.

In terms of redemption, I think it's possible, if unlikely--I think she could do it, but it would mean telling Matt that she's sorry for lying to him and corrupting him and hurting him, and it would take her leaving his life a la Angel leaving Buffy.