I agree that many posters are bitter - and from my reading of some (but by no means not all) of the PMs I have received , some are very angry towards existing JWs. (By the way , my Mum & Dad were married when I was conceived!)
I suspect that JWD posters are a "hard core" of former JWs who still take an interest in JW matters composed of a loose grouping of classic "apostate nutcase" types , many who still have family & friends in the organisation and some who feel they have found friendship and companionship (albeit via an electronic medium) with others. According to the UK map on this website, only about 30 or so Brits post compared to many thousands who have left the Truth in this country , so you are talking about a select group of people.
If you do a search on the net , you will find ex-Catholic , Mormon , Anglican and numerous other message board websites and the attitude and bitterness is very similar.
I asked a couple of long-term faders in my family if they felt bitter about their experience as a JW , but they both felt that broadly the experience was positive - they had just moved on to different interests. I suspect if they were bitter , they might have been more inclined to contribute to websites like this one.