An ex from hell is an ex-boyfriend or ex-girlfriend that is completely insane. They typically try to contact you constantly, and do NOT want to see you happy in a
relationship. They will likely do
whatever it takes to ruin your
relationships, especially when they realize that your new love is significantly better than them.
In most cases, an ex from hell is a girl. She's usually an ugly bitch who feels threatened by your beautiful, smart new girlfriend.