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.
Guy: Ew. WTF?
Girlfriend: Who texted you?
Guy: That ugly skank
Sydney, she wants to have sex with me and has no respect for our
relationship.
Girlfriend: Ugh, what an ex from hell.