Dysfunctional heterosexual anime relationships are practically a staple in the genre, so it stands to reason the person often making everything terrible is a dude. While some anime men are inexplicably boring, the worst boyfriends in the medium are manipulative, neglectful, unfaithful, and abusive. Some, like Light Yagami, even straight-up kill their girlfriends. If you're in search of healthy relationships, these are not the series you should watch.
Stay far, far away from these anime bad boys, no matter how hot they are. Men are not entirely to blame for bad relationships - there are plenty of terrible anime girlfriends who make everything awful, too. However, there's little reason to excuse these boys' flat-out abusive behaviors.
All Makoto Itou wants is to have as much sex as humanly possible, with as many consenting girls he can find. There's nothing wrong with getting constant action if you're honest about your intentions, but Makoto is really not.
Instead, he tells each girl about his exclusive devotion, then ghosts them when he loses interest in maintaining the facade. His four-timing behavior makes it hard not to sympathize at least a tiny bit with Sekai, especially when she kills him at the end of the series.
Masamune Gotou, a professional shogi player in his early 40s, has a wife in the hospital with a serious illness. He also carries out an affair with a 22-year-old girl, Kyouko. If this scenario sounds bad on its own, it gets worse.
Gotou treats Kyouko with shocking disrespect, regularly insulting and hitting her. He's both a terrible husband and loathsome secret lover.
One of the most basic prerequisites for being a decent boyfriend involves not offing your significant other. Light Yagami tricks Kiyomi Takada into thinking he loves her, then executes her to help mask his identity as a mass killer. Light's initial relationship with Kiyomi in itself is another strike against his character - when he starts dating her, he's already with Misa Amane, who never agrees to an open relationship.
Light's priorities do not include being a good partner. Without loving either woman or anyone else, Light easily manipulates his targets into thinking he loves them. Consequently, the misled women end up willingly assisting him in becoming a god. He couldn't be a worse boyfriend if he tried.
Possessive and controlling are hardly desirable traits in a boyfriend, but they certainly describe Takumi Ichinose. He expects his girlfriend (and eventual wife), Hachi, to stay at home and keep house for him while he goes off with his band, sleeping with whomever he wants.
While Takumi is occasionally sweet to Hachi, he also constantly yells at her to shut up, insults her intelligence, and generally makes her miserable. The fact they actually marry by the end of NANA comes as a true disappointment.