sometimes, I wish these protags would just accept the bad person's proposal to marry. End that whole thing there, change them from the inside(literally).
Well, in the absolute as is case there's no reason for the protag to accept and help spread whatever 'bad' thing they're doing since protag as the consort would have less power.
And if the 'bad person' can be reasoned with/have reason that protag would agree to help them, that brings up whether they are really 'bad' in the first place.
like,
Bride of Barbaroi starts with him proposing to her after kidnapping her, and if anything she's the one who changed after learning about the bigger world.
or what if when the protag's side treat them even wrose.
any story where the prologue is about how the 'hero' is basically exploited as living weapon by the country and after the latest, seemingly fatal mission they instead get married to the big monster they just fought (demon queen, dragon, etc)