1. You left before any of your stock vested. Assuming a standard four year vesting schedule and a one year cliff, this makes complete sense. (Although you are saying that you paid for your options, so this likely isn't the case here).
2. The company got shut down, its assets liquidated, and the founders and a select group of employees went to work for Google. This is perfectly sensible from a legal/operational standpoint, but is really unethical if the founders didn't take care of existing stock holders.
3. The founders had liquidation preferences that employees didn't have. This seems really unethical to me, but it's also something you could have seen when joining the company by doing the legal diligence.
Bottom line here is do the diligence, understand the mechanics so there is no confusion as to what they are later, and, most importantly, make sure company founders are decent people who'll pick the high road if/when presented with a choice. I don't think I could live with myself if I had to look in my fancy, gold-plated mirror every morning knowing that I screwed people that trusted me to lead them out of what's rightfully theirs. Make sure you get a good vibe from the people you work for - nothing can substitute chemistry, character judgement, and basic human decency.