
I have read this on artlapsa and the manga uses the brainwashing trope. It's implied that the ML has been brainwashed by the saint. He reincarnated along with the MC ten times. He is sane for a short time after regression but loses his consciousness each cycle, and only manages to snap out of it during MC's execution and repeats the cycle.
I suggest to read this as mystery rather than romance.
He's truly a cosmic eldritch horror, isn't he? He's not showing his human face directly to her for a reason and he probably erased her memories because she was almost successful in escaping but failed at the last moment. The fact that she calls him a bastard, I wonder if that marriage is a forced one hmmm