Bunny boilers is a reference to the film Fatal Attraction which depicts a woman so possessive of a man that they are a danger to friends, family or even children they might have because 'he cares more about them than me'

If you watch anime, this is better shown by the Yandere character template. They are extremely dangerous people to meet that even without interacting or socialising with, can form dangerous parasocial connections to and engage from anything from stalker to homicidal tendencies to their focus of attraction.