Friendship is a powerful theme in the world of anime - but that doesn't mean that every anime character is actually good at the whole friendship thing. In fact, there is an abundance of abysmally bad anime friends who deserve to be taken to task for their terrible behavior.
Who are the worst anime friends? Some of them have done such dramatically awful things that there's no possible way they can ever make up for them. Griffith from Berserk is a great example - he dramatically betrayed his friends by sacrificing most of them in exchange for god-like power. Other terrible anime friends may not have committed crimes against humanity, but are still guilty of interpersonal cruelty. Eriri Spencer Sawamura of Saekano: How To Raise A Boring Girlfriend might not have taken Tomoya Aki's life, but she sure did betray him by abandoning their friendship to preserve her reputation, then blamed him for causing her pain.
No matter what precisely they're guilty of, these anime friends are truly the worst.
Griffith is arguably one of the worst friends in anime history - in fact, he's a top contender for the worst friend in any piece of media ever. Griffith was a member of the Band of the Hawk before betraying them in a spectacular power grab. He got the opportunity to join the God Hand, and gain godlike power by transforming into Femto. To do this, he had to sacrifice his comrades' lives during the Fifth Eclipse. Not only does he do away with almost every single member of his former team, but he also assaults the band's only female member Casca, while mocking her lover Guts with his treatment of her. Both Casca and Guts were his friends. It is actually impossible to be a worse friend - or person than this.
Kyouji Shinkawa is one of the main villains of Sword Art Online II, so it stands to reason that he's not going to be particularly good at the whole friendship thing. He and his brother Shouichi are the forces behind Death Gun, a serial killer who targets players in Gun Gale Online both inside and outside of the game. While this is obviously egregious behavior, what's just as bad is his relationship with Sinon.
At first, Kyouji acts like a typically concerned friend. He's a little more protective of her than is strictly necessary but since Sinon is still emotionally fragile from a traumatic event, this is understandable. Unfortunately, Kyouji becomes romantically obsessed with Sinon and starts stalking her. Eventually, he breaks into her house with the intention of ending both of their lives and is only subdued with the help of Kirito.
In the end, he's arrested along with his brother - a well-deserved fate for a terrible friend and person.
Light Yagami cultivates relationships not for the sake of genuine human connection, but for strategic reasons. He forges romantic relationships with Misa and Takada so that he can use them to gain information or use them as bait. He does the same thing with his friendships, few though they are. One of the only people he actually considers a friend is L, the detective who is trying to nail him for being Kira. Though he does respect L, and actually appears to enjoy some of his interactions with him, his ultimate goal is ending his life. Essentially, Light will always prioritize his activities as Kira over anything resembling friendship, which makes him a pretty terrible friend.
Sae Kashiwagi ruins other peoples' lives like it's her job. Sae has made it her personal mission to ruin her "friend" Momo's reputation and steal her boyfriend. Sae uses Momo's tanned skin - which she obtained through swimming outdoors - as a reason to spread nasty and completely imaginary rumors about her personal life. This is apparently due to her personal insecurities. Later in the series, Sae changes her ways and becomes a true friend to Momo, but that doesn't change the fact that she was irredeemably sadistic and cruel.