I almost never post reviews online. I don't think I've ever posted one on Amazon. I hear stories all the time of authors getting into fights with people on Amazon over bad reviews.
One of the biggest problems authors suffer from is arrogance. We spend hundreds of hours crafting our babies, pour our heart and soul into them, and then when we're done we bravely let go of their tiny little hand and watch them wander into traffic.
It's understandable that some of us would go all Dumbo Mum on anyone who might dare critcise our little darlings after all the work we put into giving them life.
Writing is hard. But responding to negative reviews and challening people on their opinions is wrong. It's unprofessional. Risky. Part of being an author or any kind of artist is learning how to accept critcism. Not everyone is going to like your master piece. Not everyone is going to see it through the same rose tinted glasses you have on.
Writing a bad review online is one thing...being a troll is another. If you read a book and don't like it you are entitled to write a bad review. Be fair and honest. Tell the author what you liked and what you didn't like. Don't take it one step further and start harrassing the author on social networking sites. Don't flood their inbox with hate mail. What are you trying to achieve?
Is it okay to bad mouth books online? Yes, to a degree. But when you start attacking the author, stalking them, or making their life miserable then you have over stepped the line. Pick your battles. Move on. There are bigger and better fights to pick in this world than launching crusades against authors.