Okay, so this is a bit of a rant so it'll probably entail me using a lot of caps and doing a lot of rambling. You can just look the other way if you don't like it.
When writing a bad review there is particular etiquette to follow. I find it's pretty unspoken although I did actually recently see a post about it, I didn't actually read it but it caught my eye. I haven't actually had to write a bad bad review yet because when it comes to books, I am easy to please. Although, there are quite a few things to remember when writing a bad book review.
One being that it is just your opinion. Purely your opinion on the book and whatever you find wrong with it. You may hate a book that I love and it because of we interpret things differently and build different opinions. It makes us unique and interesting. I actually like to read reviews from people who've felt differently about a book because they usually mention things I hadn't even thought about. I think it's intriguing. We all think differently and different things bother us or stand out to us and so we form our own opinion.
Two, there are people who will like the book your talking about. No matter how crappy or rubbish you think a book is, sure enough, someone out there will like it, even love it. Keep that in mind because you don't want to offend those people. There is a fine line between an opinion and being plain rude for example, saying something along the lines of 'this books seems to be written for those with a more simple mind or a younger audience' is sharing an opinion, something you noticed. While saying something along the lines of 'only stupid people will like this book' is rude. Opinions are all well and good as long as they are executed with manners. Book reviewers aren't here to offend.
Thirdly, and probably the thing I have the most to say about, don't author bash. Authors work very hard, I want you to try to write an actual novel the next time you find yourself bashing an author. That crap is so hard. They have so many drafts, they do such much editing, so much writing. It really is a lot and it is so much more than I can ever fathom. Don't bash an author just because you don't like a book. It's probably good in terms of writing quality plus with the amount of time whichever author probably spent working on it before it got published, their book is probably like their baby. I just hate author bashing so much because they work so hard and liking a book has a lot to do with matter of opinion more than literally anything else. Just don't do it, WRITING IS HARD, we should be congratulating authors for being able to write that much plus it being good enough to make the publishing stage.
Basically, be mindful of the type of things you say when reviewing.