Okay, so let me check with you for a sec so I know exactly where you stand.
What you're saying is that you would be okay with things if Keyaru was portrayed as just as "awful" as his former abusers and his fixation with paying evil unto evil unto evil as a side effect of the terrible abuse and betrayals he suffered breaking him? Kinda like Raoul from Fukushū o Koinegau Saikyō Yūsha wa is.
Or are you suggesting there should've been a "better" way to gain the use of the princesses' frankly indispensable skills?
What I’m saying is that someone who brainwashes, tortures, rapes, etc like the MC is just as evil as the people who did those things to him regardless of whether it’s “revenge” or not.
Would the princess be a righteous person in doing the same things to the MC as revenge if she ever returns to normal?
I actually think Raoul is a great counter-example to Keyaru. Raoul is an evil person in both the writing and artwork. He is willing to use deceit to trick people into falling into his traps and happily subjects them to horrific tortures in both life and death.
And he does all of that while being completely aware that many of his main mortal targets were bewitched by the gods to act abnormally.
The reason why I think Raoul is a much better character than Keyaru is that Raoul is self-aware of how awful he is. He isn’t going around collecting a harem and talking about wanting to stop others from doing the same things he is doing.
Raoul wants his revenge, and he doesn’t delude himself into thinking he is a good person. He isn’t written as a good person.
Keyaru wants his revenge while doing the same horrible things to others that were done to him, and yet he is portrayed as a good person.