I'll give out two examples of characters who come close to qualifying but ultimately don't count for the trope because of technicalities.
Dexter Morgan: Now, Dexter is a rather stylish operator and was an undetected serial killer for over a decade. While he is undeniably bastardly for killing hundreds of criminals simply to satisfy his sadism, he has many redeeming and sympathetic qualities to counteract that. Helps that he is pitted against many viler serial killers in the show. However, while definitely close, Dexter ultimately doesn't count for what he did in Season 2. There, he was dating Rita but then cheated on her with Lila out of pure stupidity by having sex with the latter for no reason which screws him over with Rita. Now obviously, cheating on your spouse isn't an automatic disqualification for the trope as shown with Lawrence Gordon from Saw and Dexter does apologize to Rita about this which prevents the slip-up alone from being too mitigating by itself due to Rita forgiving him. However, IIRC, cheating in a relationship specifically is only allowed under three conditions:
The extra marital relationship never goes beyond sexual.
The character is two-timing purely for pragmatic reasons.
The spouse being cheated on is an unsympathetic victim.
Ultimately, while Dexter does come close to counting for the trope and even regrets cheating on Rita, the fact that his cheating doesn't fall under these conditions is what prevents him from counting as fails on all three fronts. He had sex with Lila while still dating Rita, him cheating on Rita was done for no reason other than maybe being a douchebag as he never planned to use Lila as a pawn and Rita is treated as an innocent victim. It doesn't help that he had sex with Lila seconds after scolding her for threatening his relationship with Rita for no apparent reason which I can't understand at all.
For my next candidate, it will be Kiyotaka Ayanokoji from Classroom of the Elite. Now, Ayanokoji is a brilliant chess master who is basically The Planner of the entire series. For the most part, he is pretty Magnificent as he helps support Class D from behind the scenes. However, the reason he doesn't count for the trope is the 'Spread Your Legs' Scene. In that scene, he asks Kei Karuizawa to spread her legs in a disturbingly predatory manner which constitutes as sexual harassment. Considering that Ayanokoji isn't from a kids show and that Kei is treated as a victim, it's safe to say that this makes him too vile to count.
You can give your own examples below.