FPH active subscriber here. Can I try with an answer?

Personally, I hate my ex-self. I was fat, not obese (or anything you can see daily on that sub) but chubby. I never want to go back to that, and FPH is a great reminder how disgusting that was. Because memories fade. And I was fat, lazy, unmotivated, unhealhy (both physically and mentally), deeply insecured, anxious all the time, unorganized and without any discipline, uncomfotable in my clothes and with myself. And more. Especially considering that I live in a country where young people are not fat and being fat is not a norm whatsoever. I hate those two years of being an idiot who could do something like that to himself more than anything.
One and most common thing the richest person in the whole world can't buy is - time. And those two years were the completely wasted time of my life that I'll never get back.

A good amount of FPH subscribers are like me. Ex-fatasses. And that's why we hate fat people with a passion. They remind us of ourself.
Then again, there are many of them that were never fat in their life. And they hate fat people too. Why you ask?
Now would be a good time to mention there are two types of fat people, like all people - self-aware and delusional. I was self aware enough to understand how pathetic I was to change it. But there's nothing worse and more annoying than a person with flawed mind and body that's completely oblivious to all that. Not only shameless, but it has enough courage to preach and invent stuff like HAES, and run whole campaigns about being normal, healthy, (sexy? ugh) to have sacks of lard on your body.
And that's why not-ex-fats hate them.
It's an apropriate counter-measure to something that fatasses started first, and it's only natural respomce that someone will call bullshit to those campaigns and eventually form into a group of people that think the same. And that group is called Fat People Hate.

And I believe there's a third group of people, not ex-fats not so against organized bullshit, but wronged by a fatass somewhere in life, victim of a fatlogic. And now they hate fat people to.
And it's very easy to hate a moving lard that we call "people" when we're in a good mood. Not only it looks like a digusting abomination, but it's almost always followed by a large set of mental vices.
Basically they're like big children, like kids in a grown man/woman body, without any self control. And we can forgive children for acting like, well, children. But someone whose supposed to be a grown up... ugh... god I hate them...