What you are hitting on is one of the most complicated aspects of this case "did he know how bad it was?".
One could answer by this age he should, in fact you can tell he knew somewhat how bad it was to commit such an act, but what you are missing is that Chris usually holds very one note definitions of things. To him rape is ONLY sexual assault, violent and with one party crying and begging to the other to stop, I dont think he considered this rape, I think he truly thought it was ok because she gave him """"consent""". (Which given that she has dementia doesnt mean much)
Another answer could be that he truly thinks himself as above the law, above what you and I perceive as too far, as you said he is used to wrist slaps, so whats one more? I know we state he could never kill someone, but I think with his mindset and how he has been raised he probably thinks he could have gotten away with it, its not something that is out of left field with him given his angry and violent tendencies. Remember in his mind he truly believes himself to be the hero, all the Sonichu comics and fan fics he wrote where he wins, he truly believes that because he was raised to think he was that. This comes down to a theory I have had for years about "main character disorder" but, with Chris he's gone beyond that into a legit delusion.
Or he could just be a fucking retard who knows?