Is the Product Owner a Villain?

Short answer, yes. Long answer, still yes.

The PO hasn't has to be a villain, only look sometimes like one in front of her team.

Why such a nice gal like the PO has to look though? Simple, if she doesn't, team members will end up imposing whatever they believe that the user should need from a system rather than to hearing to user's requirements.

By the same token, as a user's representative, the PO is in charge of sometimes say "your work is not what what I need, I can't sell that to clients, you need to redo it". The team will be close to kill the PO but in the long run her firm hand rejecting not good user stories or asking for work to be redone will be key for taking the ship to good harbor.

Dirty work PO's it is, but somebody has to do it. May the force guide your steps POs.