NTR and this is a bit different, when you read NTR, you know that it's a hentai. You know either the wife/gf raped or cheating gonna happen.
You don't really feel that bad because you know what gonna happen and you know that its that kind of manga, you rarely get attached to hentai characters.
But when you read a non hentai manga, you got a little bit attached to the characters, so when something horrible like this happen, you're gonna feels so shit and its harder to overcome than when you see something horrible happen in NTR hentai.