If I were God, I would set aside a special place in purgatory for every anonymous armchair critic who posts an utterly unfair book review online after giving not 2 seconds of consideration to the author, their actual work, or how that unfair review could negatively impact them. After that now-deceased critic arrives in purgatory, he/she will be ushered into a small, dark room illuminated by a single flickering light bulb, with a small wooden desk and chair, a rusty old typewriter that is missing the letter “r”, and a ream of paper. And he/she will be told:
“Now, for every book for which you posted a scathing, manifestly unfair review, you will write a better book. If you posted 39 such reviews, then write 39 better books. Oh, and don’t look too defeated. Heaven will be waiting when you are done.”
I am not God, of course. That said, I suspect he has some ideas of his own. So before you post, put yourself in the seat of the author and review their book the way you’d want your book to be reviewed.

