One of the most dramatic moments in any anime is the moment when a character takes the life of a family member. Sometimes, it's a total accident - Goku bore no ill will towards Grandpa Gohan and would never have crushed him if he'd had control over his Great Ape Transformation. But at other times it's totally malicious - Kurou Sakuragawa's grandmother absolutely meant to feed her grandkids mermaid and kudan meat in the hopes of giving them powers, and she didn't care if it destroyed them.
Which of these anime characters who killed their family has the most tragic story? That's up to you.
Acting on the orders of the manipulative Danzo, Itachi Uchiha massacred his entire clan, including his parents. He genuinely believed that this was the only way to secure peace for the village and save his younger brother Sasuke's life. He does save Sasuke, and because Sasuke has no idea what Itachi's motives were, he swears vengeance on him.
When she was a toddler, Eri's quirk, Rewind, got out of control and ended up rewinding her father so far backward in time that he no longer existed. She had no malicious intent, but her mother still responded with fear and left her in the care of her yakuza relatives - and ultimately the villainous Kai Chisaki.
When Goku was in his Great Ape form, he accidentally steps on his adoptive grandfather Gohan, instantly ending his life. Because of his young age and his lack of awareness of himself while in that form, he doesn't find out until he's an adult that he's responsible for his grandfather's passing. Luckily, the old man doesn't hold it against him when they meet in the afterlife.
After accidentally using his Geass on his half-sister Euphemia, Lelouch is forced to take her down or watch her massacre a stadium full of the very people Euphemia most wanted to help. Her demise is one of the most tragic moments in the whole show. Lelouch - and the audience - may never truly get over it.