Idiots wanted to buy fake money and got burn. Ponzi scheme. Bubble burst. They bought something that doesn't exist. It's going to 0 because it's worthless! There were a few saying that it's the best time to invest if you thought about it during the bull market and thought the price was too high, but those posters were downvoted and ridiculed.
I would have been singing in that chorus back in 2012 or so when bitcoin seemed like a ridiculous form of electronic currency that was completely unnecessary. It reminded me of that digital currency Whoopee Goldberg used to promote (Flooz) back in 1999 or so. People thought the internet somehow needed its own form of money. Nobody ended up needing it or using it, and it died. It took me a while to figure out how bitcoin is different, how it's P2P and not under the control of any corporation or government. Some people still don't understand this, but as far as the world
needing bitcoin, I can see their point. Right now I don't think we do, but if there came a time when fiat money suffered a severe crash, it'd come in pretty damn handy. It's also useful for online anonymous transactions.
The other things you mentioned also happen in the stock market. When prices are up, people think it's too expensive to buy; when the market crashes, they get scared and don't want to buy something that's obviously on sale. Even worse, if they sell at a loss when the stock they own starts to plummet. The markets are a weird taste of human psychology.
I do not know what these people doing on bitcointalk forum if they did not believe in crypto future, especially these people which saying that Bitcoin will go to zero.
A lot of them are trolls, though I'm not sure why you'd have an account on bitcointalk if you really didn't believe there was anything to bitcoin. My guess is that at least some of them are bitter about losing money at some point, and they're trying to spread the misery as far and wide as they can.