So many people have already talked about this, but I'm going to put in my two cents' worth too. Book reviews. Book reviews are not for the author. They are for other readers. They tell us people's opinions and why they thought that about the book. Whether it's a one star, a five star, or something in between, it's to let people know the writer's opinion. It's not to bully the author or badmouth them.
Then we get to the cyber bullies. The ones who post one star reviews that blast the author as an individual. They threaten physical harm or post personal insults to the author themselves. They are there only to drive down the author and damage his/her ranking online, to injure their self esteem, and to bring others into their damaging mindset.
There are also author bullies. They are the ones who go batshit crazy about a negative review. They go off on the reviewer and start a flame war that leads to people chiming in about the author's bad behavior. This just leads to more drama from the author to the point where people are put off from their books because of this behavior.
The truth of it is reviews can lead to many different things. We need to stop and take a look at what we're doing. Are we reviewing the book because the author insulted us in some way? Then we probably shouldn't post it. Are we reviewing it honestly because of our opinion? That's a little better. Do we disagree with another reviewer's opinion? We keep our mouths shut and respect that it's their opinion. The only time we intercede is if it's a bullying opinion. Then on Amazon we can vote it down. Goodreads we can report it (as far as I know...I haven't been on there in a while because of the cyber bullying). There are ways we can bring these to the attention of those in charge of the website and hopefully someone will take care of it.
To summarize, be respectful of the author but don't be afraid to voice your opinion. Make sure it's on the book and not on the author. Authors, don't answer reviews. Keep your mouth shut and let your readers speak their minds. If you see someone cyber bullying another person, report it.