"Every single toxic person I've met can switch it on and off when they please"
Yes, this is a great point, and true of the three toxic people I referred to earlier. It was one of the aspects that finally pushed me to lose tolerance for it. However much they protest they are the victim, and can't control/are not responsible for eg screaming at you, they don't scream at other people randomly. In fact they are quite conscious of reputation management.
I'll never forget my MIL having a seemingly uncontrollable panic/screaming meltdown at my DH (this was a weekly occurrence, any time anything was going well, or there was attention away from her) this time, It was for something totally random, like she'd lost her keys and he was trying to get her to think calmly about where they could be. She was screeching at him hysterically, calling him Hitler (this was one of her favourites, along with saying she was like Jesus on the Cross, that she wished she was dead, and that he'd ruined her life from being born)).... anyway, mid flow, a friend of hers calls. She answers in a nearly totally normal voice, then starts screaming when she puts the phone down again.
She, and the other two people have a string of broken relationships, professional positions etc behind them. They end up very isolated, but it's always everybody else's fault. People just don't understand.....