It's important to assign credit where credit is due. Especially when it's the villain's descent into villainy that is the thing being credited.
An important literary trope occurs when the hero tragically causes the downfall of the villain. If you can set this up in a game, it will make any confrontation all the more poignant - but you have to make sure to point it out lest the tragic irony be overlooked.
Or it could all be the bad guy's delusions. Still, it's good to make the heroes think.
Darth Vader: Do you like the changes I have made?
Obi-Wan: You were a good person once. I still sense it within you. The love in your heart.
Darth Vader: You killed the only person I'd ever loved.
Obi-Wan: No. You killed her.
Darth Vader: That's not true.
Obi-Wan: You doomed your love from the start.
Darth Vader: No! It was you! Your meddling, your interfering! We could have been happy together!
Obi-Wan: So like you to blame others for your flaws.
Obi-Wan: Seriously, is striding around in a black mask and cape, Force Choking your underlings, where you saw yourself twenty years ago?
Darth Vader: And whose fault is that?!
R2-D2: Actually, like everything, it's all kind of Qui-Gon's fault.