@SailusGebel He did have one knight which was his half-brother and African King that is suspected of being gay for Lancelot. Who as you should already know, cucked Arthur.
On the other hand, Arthur did have a yandere Fae sister who was so pissed that Arthur married someone else that she birthed a child from Arthur's seed named Mordred as a form of revenge until she forgot why she was mad and told Arthur about the NTR going on behind his back.
If I remember everything correctly, almost every knight did some really heinous and atrocious deeds, so I doubt this counts as a shonen. More like a grimdark anime, a true one, unlike the wannabe goblin slayer.
@SailusGebel depends on the knight and what era of story-telling was going on. Most of the knights were considered incredibly sincere, kind, and honorable people. There were a few outliers though. One was a werewolf, another used magic and was nearly burned at the stake, Lancelot cucked Arthur, Mordred tried to overthrow Arthur, the list goes on but most of them didn't do anything evil or heinous.