The problem is, that if I remember correctly, in her memories of her past life he told her that he only married her because his father made him after being very rude to her for just making him tea. I dont know if he lied or not, but that was part of the reason she became so bitter and turned out even worse (I'm not justifying her behavior thought). So technically, even if he doesnt know it, it's his fault that she thinks he hates her and doesnt want to marry her. And if he felt any pain when executing her he should have known he had contributed to the situation (once again I'm not taking any blame off from her, she made her own choices but he influenced her too).

What if the prince actually loved Elise and when he executed her he was suffering a lot and didn't want to do it? That should have been terrible!