There are two versions of Minthara, the one we got now is just another victim of the Absolute and like mentioned by Cromsonrider, hates unecessary evil only goes for it when its necessary to gain power to overcome the cult.
It is messy and then there is the legacy version of Minthara who is for pure evil, example of legacy dialogue is
when she breaks up with you for denying your father's power as a dark urge, where she wants you to take control of the Netherbrain to rule over others and while that is still possible to do in the current version of the game, some of her legacy dialogue are turned off if you disregard certain recent bugs that enabled one by accident such as the break up.
You might feel that way about MInthara because there are some legacy dialogues that makes her seem like all out evil as not all of them are disabled, ultimately its your game and you choose how you wish to perceive her as but what triggers her romance is just reaching high enough approval rate and that's it, differences you have for how you choose to recruit her is missing a romantic scene from Act 1 and let the game flag you and Minthara to be in a relationship but you still need her approval raised high enough to get certain dialogues that suggests a romantic evening "I am completely yours tonight" moment.
In other words its based on approval rate, if you wonder why I suggest two versions of Minthara is because when the epiloque patch hit, you could branch off to a different path with Minthara and the only path that lets the two of you reach the epiloque chapter which means not going all out pure evil.