I don't know the exact disorder (or if she even has one) because I'm not a professional and she hasn't been diagnosed, but here's my experience all the same. My sister, at around 4-5 years old, rapidly became extremely aggressive to everybody. My parents dismissed it a kid misbehaving, but I'd been bullied for years in primary school already, so I kind of knew what 'misbehaving' kids looked like, and NONE of the kids that picked on me had the relish my sister had when she was tearing you apart. The simplest way I can differentiate it, is by saying that kids who picked on me did so because they thought it was funny, whereas my sister did it for pleasure. She truly, deeply, just delighted in causing people pain. Whether it was physical or emotional, she enjoyed hurting people. She’d routinely bully my mum to the point of tears, and you wouldn’t believe the looks of pride and satisfaction my sister would have on her face when she knew she’d broken someone. When you started crying, or became visibly upset, the point where most people back off, that's when she really got off on it. That's when the fun really started for her. If this already sounds cartoonish, that's because it was. We had a neighbour who was an extremely shy, quiet, sad girl, and my sister just used to dominate and manipulate and bully her all the time. She literally used her as a maid for months. She'd just come around to be bossed around and used to wait on my sister all day until she went home. I used to tell my parents about it, and they would say, 'Oh no, she likes it. They're just playing a game where she's your sister's maid. It's just a game.' -__________- Anyway, my sister stabbed me with a fork because I got ice cream before she did one night. A metal fork, that literally was stabbed into my back. She never apologized and my parents never punished her. Things like that continued to happen my entire life. She used to throw my dog down the stairs if she ever got angry. She also used to hold him hostage, and threaten to beat him if I didn't let her watch what she wanted to watch on TV. So I'd always give her the remote, and then she'd hit him anyway, with this huge smirk on her face. Every fucking time. So by the time I was 8 or 9, I was telling my parents that something was seriously wrong. Like, more than just a kid being mean. Just because she wasn't stabbing people to death and eating their faces yet didn't mean that something wasn't fucked up. She really, really had sadistic pleasure in hurting people however she could. My parents ignored it, and when they didn't ignore it, they blamed me. They said I'm her older brother, and any bad behaviour she has, she must have learned from me. She’s 20 now, and in the last few years, she's been arrested for repeatedly assaulting police officers, (she did this every 2-3 weeks for literally years before they actually decided to charge her), her last boyfriend filed an AVO (restraining order) on her after she assaulted him, she's assaulted me repeatedly (in childhood and adolescence), and my mother (who was nearly 50 at the time) she's got serious issues with alcohol, having repeatedly ran through the streets naked and screaming (all while my parents watched and refused to admit there could be anything wrong with her). I don't know whether it's narcissism or sociopathy or psychopathy or borderline or fuck knows what anymore. I begged my parents, I fucking begged them, to take her to a doctor, or a therapist or something, because this wasn't normal and aside from not wanting to be fucking attacked every single day of my life, I wanted her to get help too. As mad as I am at her, and as mad as I was, if there truly was something psychologically wrong with her, she needed the help. Plus, it’s not even just about us. She’s going to go through the rest of her life, fucking with people, and hurting them, and manipulating them, and assaulting them or worse, and every day that we didn’t do everything we could to try and stop that from happening makes it more and more our responsibility, and our failure. My parents just kept saying if there actually was wrong, it'd screw up her employment opportunities. I don’t know what disorder she has, or if she even has one. But I swear, at some point in my life, I'll turn on the TV or read a newspaper, and there'll be her face on it, because she will have murdered her partner, or drowned her kids in the bathtub or something (she doesn't have any kids yet, thank fucking god, but you get my point) and everybody will cry and say nobody could have ever seen this coming, and it will be complete bullshit. Never underestimate the power of denial. Some parents would set themselves on fire before admitting there might be something wrong with their child. For fucks sake, if there are any parents reading this, who on some level know their kids desperately need help, it is a THOUSAND TIMES WORSE TO DENY THEM HELP. If they’re already this bad, imagine how much worse they’ll be with another decade or two without some kind of help. tldr; denial is powerful.