I told my Girlfriend I'd rather have her dead rather than seeing her with her ex?

So we had a little fight and i know i kinda hurt her feeling and she is mad because i said i'd rather have her dead than to see her with her Ex long story short story in my point of view i was trying to mark my spot and i know i could have possibly hurt her feelings but there was just no way other way to express myself for i myself hate her Ex i won't tell why i'll just keep this way and please i need females tips on what to do and help me understand why does she feel like it's not okay to have a grudge and i might be stupid but i don't get it tho in her words ''Im stupid''