I would like to put forth the radical notion that, if a fat person is fat by choice, it's okay.
I'll give you a moment to sit with that idea—that it's okay for someone to choose to be fat. Because it really is a radical notion, and, like other radical notions, it is both has the capacity and is likely to evoke visceral reactions of protest. Like: "But being fat is (potentially) unhealthy! And that's not okay!" But, if you give yourself a moment or two, you'll probably realize there are other potentially unhealthy things that people do, which you would probably argue in favor of allowing them to continue doing.
It's more dangerous to ride in a car than be a pedestrian. But if a person capable of walking to the store wanted instead to hop in their car to pick up milk a mile away, you'd probably think that's okay. Because, hey, maybe they have a good reason for preferring to drive.
More people get hurt jumping out of airplanes for fun than get hurt gardening for fun. But if someone prefers the adrenaline rush of skydiving to the relaxation of gardening, you'd probably think that's okay. Because everyone's different, right?
Sometimes, doctors tell patients that a surgery, or an experimental treatment, or a new drug, might actually be more likely to kill them than cure them. But if someone decided to opt for the risky cure, you'd probably think that's okay. Because it's that person's body, not yours.
So maybe it's all right for you to think it's okay, if someone chooses to be fat, rather than thin.
Because, the thing is, holding in judgment people who are fat by choice doesn't make a whole lot of sense, given our general tolerance for all sorts of things that people do which carry with them risks to their health (like being born, or giving birth, and things way more controversial). And people are going to be fat, or not fat, irrespective of your judgment about fat people. Letting go of fat hatred won't change anything—except, of course, to make the world a little bit better a place for its fat inhabitants.
It can be a hatred that's hard to let go of, even for fat people, because letting go of that hatred, and replacing it with acceptance, can feel akin to giving fat people permission to be fat.
But being in the position of feeling like permission is yours to give is a manifestation of privilege. And maybe it's all right to let that privilege go.
Maybe it's all right for you to hold the position that if a fat person spends hir days as a walking stereotype of a "bad fatty," eating mass quantities of unhealthy food, that is hir right.
Maybe it's all right for you not to draw a distinction between "good fatties" and "bad fatties," even as you recognize not everyone is fat for the same reasons.
Maybe it's all right for you to consider that if a fat person is spending hir days eating mass quantities of unhealthy food, it's none of your business (unless zie invites you to make it your business) whether zie is doing so because zie has an eating disorder, or because zie has an addiction, or because zie is self-medicating with food, or because zie is insulating hirself from abuse, or because zie is creating a barrier of flab against real intimacy, or because zie is bored, or because zie is self-destructive, or because zie has no will power, or because zie is just a gluttonous foodie who loves the taste of rich foods.
Maybe it's all right for you to acquiesce that you cannot tell just by looking at hir for what reasons zie is choosing to be fat, or even if it's hir choice at all.
Maybe it's all right for you to treat fat people with dignity either way—and let fat people sort out for themselves the business of their being fat.