The thing is, if you're having a fit because you're epilectic, then you can't help it, and a word like "epi" should obviously never be thrown against you.
But if you're having a fit because you're a prat, then being called 'epi' would mean you could do better than to behave like someone who has a very good reason for not being able to control themselves.
Epi is doesn't have to be derogatory towards epileptics, it's derogatory towards people who don't practise self-control when they perfectly well could.
Ditto for words like dumb, spaz or retard -- those words shouldn't cause offense when used about someone who isn't mentally disabled/having cerebral palsy/mute. It's how the word get used that matters, not the word itself.
It's the reason that black people sometimes use the N word with each other -- it means, "You're being an ass or at risk of it, living up the worst stereotypes of OUR people, so get your act in gear and do better". Whereas when white people use the N-word, the historical power imbalance/true racism gets implied. One of those is mildly provocative, the other usage is outrageous.
-- Speaking as someone who never uses any of the above words, but I feel like I am on eggshells about what I might say nonetheless.