That snarky putdown that appears on your blog. The commenter who seems to have nothing better to do than to systematically disagree, obnoxiously, with every point in your latest article. The attention-span challenged reader who skimmed the post and wrote a critical comment that doesn't even relate to what you wrote.
These are the vampires of the online writing world. They suck energy out of you and make you second-guess even your best ideas.
1) For the truly negative comments that have no redeeming value: recognize that there are condescending pricks out there who get off on criticizing others. If you receive a harsh and unfairly critical comment, recognize it for what it is, smile and move on. Don't allow your brain to fire any more neurons about it.
2) Not everyone's opinion carries the same weight. The person commenting may have no expertise whatsoever on the subject you've written about. Assess the value of the comment: if there is no "there" there, repeat the instructions from tip #1 and don't let your brain fire any more neurons.
3) At times a negative comment may indicate that you've miscommunicated your ideas. This can be extremely valuable criticism. If the commenter misunderstood what you wrote, then perhaps you need to re-evaluate your article for clarity, or perhaps you unknowingly implied or suggested ideas that you didn't intend.
It's also entirely possible that this commenter misunderstood you because he or she didn't actually read what you wrote. In that case, repeat the instructions from tip #1 yet again. If a reader isn't willing to fire any neurons before commenting, you shouldn't fire any neurons in response.