Lemme get this straight;
- the MC has to spend vitality/lifeforce on Kania because the dark mana she was born with is eating her own lifeforce
- if one of the heroines finds out that the MC is just pretending to be a villain and that he's actually a good guy, he takes a vitality penalty
- if all heroines were to find out, the penalties would equate 100% of his lifeforce, which would kill him on the spot
So... why doesn't the MC just come clean to Kania, eat the lifeforce penalty, and make her his accomplice? Sure, it would take some effort to convince her - but that still beats having to spend tons of extra lifeforce on her just because she keeps trying to murder or sabotage the MC.
Kania is already his butler anyway, so it wouldn't even stand out if she'd become his accomplice and be seen doing chores for him.