When I used to hate my sister. Literally HATE her. I enjoyed her being in pain all the time. I never inflicted it, because I'd obviously be punished, but I felt her pain was sweet justice for what she's done to me. I fucking hated that little shit. I was happy when she got diabetes. I was happy when she broke her arm and wished I was there to see it. I was happy when some stranger called her fat and she cried. I hated her and was VERY happy when she suffered.
Why I hated her: She would get me in trouble by lying and I'd be locked in my room with literally nothing to do but sleep, and have lunch and dinner for weeks at a time. Because of her lies I'd get whipped with a belt, and she'd stand there smiling at me. Which happened about 3 times a week. She sat outside laughing at me once after my mom punched a hole in my closet and whipped me. If I actually did do something bad, and there was proof, the punishment was the same but 10x's worse. My parents (mom and step dad) thought she was gods gift to Earth. When my sister often hit me so hard with metal shit I would bruise or bleed, I'd get in trouble for making her cry from screaming in pain. I would spend 6+ hours on art, or a snowy hill outside to sled on, and she would either destroy it or take it with my parent's permission while I got sent to my room for the next day or so. She would lie to my mom about things my grandma would say and not let us see her for months because of it. I was grounded 80% of my childhood because of her, and had nothing in my room. No TV, no games, no books, I had my bed and dresser. Her room was filled and cluttered with all the things I never had. School would have been my outlet except I had no friends and got bullied. I would read at recess. I was around 8 when I started contemplating suicide. I absolutely hated her and my mom. I became numb to punishment by the time I was 10.
I ended up being sent to live with my dad which was the best thing to ever happen to me. I don't hate her anymore and we're friends now. She's so different now I can't consider her present and past self the same people at all. She's sorry for what happened too. I barely talk to her anymore. And I DO have empathy for any hurting thing. But I still have no empathy for her or my mom. My cat was the only thing that made me happy.