Mayadere - a dangerous form of Tsundere, replacing typical annoyance or comical violence with outright murder, not obsession, just plain murder.
A mayadere usually starts as an antagonist trying to kill the someone but switches sides after falling in love. This character is unpredictable and dangereous for everyone including the romantic interest.
Examples:
Elizasleyn - Kamidori Alchemy Meister
Ilyasviel - Fate series
Saya - Little Busters
Kurumi - Date A Live Rinne Utopia