Do all former mormons despise the church? This is an obvious no. Just like there are those few people at church that are rude, and ignorant. Most people at church are pretty nice, but the jerks take center stage because they are the loudest. Since the exmos that don't hate the lds church are pretty much silent, it's the ones that do, and speak out, that you hear.
Why do people who leave the LDS Church have such angry feelings about it? I can't speak for all exmos, but I'll give you my opinion.
1. I hate that I was lied to. About church history, about the restoration, about LDS Church history in general, about the doctrine, and about just everything. So many lies, changes, edits, and word twists that the truth is so far off what is being taught. And their answer? Just have faith that it's true. So when you love something, and devote your life to something, and find out you were duped. It hurts. Like getting cheated on by someone you love and trust X1000. It truly hurts.
2. Their are two LDS Churches. The church for the members which is a show. A playright if you will. And then there is the LDS Church itself which is a corporate structure that stays in business by putting on that show. It exists to make money. (Wanna test it? Just go to your bishop, and tell him you can't make the mortgage payment this month. Watch as your social status goes from a 9 to a burden.)
3. They hold your family hostage against you? Families can be together forever right? Sure, so long as everyone behaves, and follows the rules. So families are pitted against each other. Which comes first, The LDS Church, or your own family? Sadly, there are so many people who have been disowned, and are without a family. Christmas is lonely.
4. We love our families too, and to watch them get taken advantage of, and exploited for quite a bit of their free time, and 10% of their income is tough to watch. Sure, we try to warn them, and show them, but they view us as angry exmormons that despise the LDS Church. To them, we're just angry, and shouldn't be listened to. Would you warn your family if you knew they were being taken advantage of by a con artist?
5. People are excommunicated, and ostracized for studying LDS Church History. They throw good faithful members away for being studious. If you learned something about Nephi that blew your mind, wouldn't you share? OMG, did you know that Nephi Parallels Daniel from the Bible? They might have hung out. (Old rumor, but I'll keep non-anti-mormon just for you. ;)
Those are just off the top of my head, but I could research a little more for you if you really want specifics.
The fact is, we're NOT angry. We're hurt, and we see other people being hurt. And that hurts. The most frustrating thing about it too, is the people we see being hurt think they're being helped, and ask for more.