I think he was corrupted by Sauron because of the way he already was. I think he was already power hungry, greedy, probably racist against non-numenoreans and corrupted in many other ways. He also usurped the throne from his cousin and forced her to marry him so he already was a jerk. Only thing missing was turning to the worship of Melkor.

Villain is a strong word. He was a corrupt man with bad intentions who decided to defy the biggest rule the Valar gave him leading to the deaths of almost all the Numenorians and the destruction of Numenor itself. So yeah, I'd say in the end that is pretty villainous. Sauron only stirred up what was already in Pharazons heart, Sauron didn't plant it there.

I agree what others have said. It's like of like the "super serum" from Marvel (I know, different fandom) because it enhances who you are. If you have bad intentions to begin with the serum would amplify that. So, I think that, because he was already an evil man, it wasn't super hard for Melkor to corrupt him and amplify (for lack of a better word) the evil intentions that were already there. Melkor didn't have to put those thoughts and ideas there. So, he was a villainous character made into a villain by the small "shove" that Melkor's influence gave.