You were supposed to think he was an ally for the first half of the movie. So I think that's why they didn't cast somebody who looked like an obvious villain.

Handsome, likeable actors play villains all the time, dude.

@Xaxafrad That's a good point.

@Greater good That's right, I guess I was just thinking mainly in my The Holiday mindset, lol.

While it was fine to have Jude Law as Yon-Rogg, I think he should have been Mar-Vell.

Probably that role would have suited him better but I was okay with him being a villain

I would like to add Jude Law himself played Captain Hook, an antagonist, in the new Peter Pan movie quite recently.

His best villain role is in Road to Perdition. He plays a crime scene photographer who moonlights as an assassin and he's bloody terrifying in it.

I think he did a good job of seeming like an ally in the first half and then switching over. Good-looking actors and actresses play villains all the time, for example, Kurt Russell who played Ego and Cate Blanchett who played Hela.