I'm incensed to find yet another writer disturbed by some random comment out of the ether. Is it impossible for anyone to put their work out there without attracting one of the small percentage of nutters who think only in negatives?
Here's a message for these guys. If you can't think positive, we don't want to hear from you. Get it? No one objects to constructive criticism. I'll spell that: C-O-N-S-T-R-U-C-T-I-V-E. If you have something negative to say, then jolly well find the positive as well. There is not a writer alive who hasn't written something worthwhile in their offering.
And to the victims of this abomination: don't take it to heart. A negative comment on its own means you've got hold of a negative personality. This type of person thrives on carping at anyone who dares to stick their head above the parapet. Believe this - they are trying to bring you down. Don't ask why. Don't go there. Just don't let them win.
Artists as a race are a magnet for people of this type. As writers, it's one of the risks we face. Arm yourself by realising what you're dealing with and you won't get hurt.
What to watch for? "I really enjoyed/liked your book, BUT ..." Don't believe the first bit. They wouldn't have liked it if it was strawberries and cream. "You made a research error on page 57." Yeah? So what? If they are being picky, you've hooked one.
A positive personality is going to be specific. "I love your hero .... I laughed and cried..." They won't be ready to jump down your throat for small errors. They might query something - politely. "I'm interested to know if..."
So what do you do? Two choices: ignore it. Respond ever so politely and just thank them for their comments. Don't justify, excuse or get into any discussion about the point raised.
Of course, there's a third choice too, but I think it's illegal...