I'm going to call someone fat after they insulted me
Why are you calling someone fat?
I want them to feel how I feel.
It is fine to be angry at someone for insulting you, but you shouldn't fall to their level. Don't insult them back and just walk away from the encounter as the bigger person.
I agree but I'm too angry to care. I have to insult them back so they know what they're doing is wrong.
That doesn't teach them anything though. You are going to make them feel worse if you tell them how rude their comment was and how it effected you, that is a better form of getting back at them.