First off, for those that don't already know, "netori" is NTR told from the perspective of the guy taking the girl from someone, as opposed to "netorare" where the main character is the guy whose girl is being taken. What I'm looking for is netori in which the guy who loses his girl to the main character is a real bastard. Not a loser, a bastard. For instance, a guy's brother cheats on his girlfriend. The girlfriend finds out and is heartbroken, and the cheater's brother sees this and tries to comfort her. The comforting leads to love-making, and ultimately the girl decides to dump her cheating boyfriend for his brother. It could really be anything. He could be unfaithful, exploitative, abusive, anything that makes you hate him, rather than just some loser whose worst crimes are working a little too late or not performing as well as the MC in bed.
Naturally, if the target NTR'ed the girl from someone else (or the MC), this obviously fits too.