Undoubtedly Q would simply say he was above such descriptions as ally or villain. He's an agent of the Continuum, so it could be said that Q's agenda was superseded by his superiors. In TNG we know he was acting out against the status quo of the Q way of life, behaving more like a child playing with bugs. After Quinn was locked away and Q was sentenced to a mortal life as a human, and subsequently reinstated, he returned to being the servant of the Continuum. Voyager's stories involving Q turned him into a revolutionary like Quinn.
When you get down to it, Q was many things to humanity. To answer the original question of if he was a villain to the TNG crew, I would say no. He harbored no real ill-will towards the crew, just as a child has no hatred for a colony of ants when he burns them with a magnifying glass. They were simply a pleasant diversion to Q.