I don't know the point of this at all, he "loved" her so much he was scared of losing her so he raped her to conceive his child and when his wife killed herself, he tried to go for the easy way out instead of repenting and while he is repenting now, why are we to feel bad for him? Why does Ridelle have a fond attachment to a person that did everything but care for her in the slightest in her past life? Even if she's just doing this for Emilia's sake, the story seems to be opening the route of Juliet being with Oscar once again... Her rapist and her killer. Just doesn't make sense to me man.
The reason for that is, all things considered, Oscar was the only one who showed her real affection sans her maid and Ethan, though even that seems doubtful since Ethan was probably acting (though its up in the air since it seems to be diverging from the WN at this point since this whole kidnapping did not happen in the capital but back in Oscar's territory) and her maid reveals in the raws that she hated Ridelle. The title has failure princess, and the fact that Ridelle even considered herself like that, and her family not being open in their affection to the point of her becoming like that is part of the issue, and they both did love each other before they got married, and that's the reason why Ridelle even kills herself in the first place; she doesn't want to cause dishonour to Oscar. How is Oscar the one who killed Ridelle when she did it herself, out of her love for Oscar. Him saying he is the reason is his guilt for being unable to protect her.
Honestly, the marital rape, as we would classify it now, is a jarring thing which honestly didn't even need to be there. It was Oscar acting out on his insecurity, and no one thought she'd conceive with just that. It's easy for us to say that Oscar was trying to take the easy way out after Ridelle's demise, but did you forget how hard of a life he's lived?
His father didn't give two shits about him, his 'supposed' mother hated him and never did anything motherly for him, his actual mother died, he inherited a house which he largely didn't care much about and worked to keep it together since he has a responsibility to his people, and when he finally gets some happiness in the form of a marriage, it breaks down because he and Ridelle didn't openly communicate and assumed a lot of each other, as evidenced in the different perspectives we see in both their recollection of events. Then his wife and only real friend die.
I'll agree that what he was contemplating is never the answer, but I can understand how crushed he was to reach that mindset. He then has lived till now blaming himself, and in the raws, he catches up to Ethan and tells him to leave Juliet and Emilia out of this, citing the only one who deserves to be cut down is him, and he drops his blade while waiting for Ethan to cut him, so he knows he is in the wrong and has repented.
This also partially the author's fault; the story the author wants to tell is there, but they've admittedly fumbled with showing it to readers, and sometimes they've crossed into extremes when it wasn't really necessary.
Edit:
@AngelusDaeva good work on the TL. Would just like to point out a correction; the title is probably 黒幕 in JP and yes the literal reading is black curtain/black veil, but the actual meaning is mastermind. The word is a reference to how during the Sengoku period and other past eras of Japanese history, the people actually holding power would be sitting behind a black curtain which obscured them. Just thought I'd add that tidbit since I like to encourage budding TLs.