Internet trolls. With the option of anonymity on the internet, so many take it as an opportunity to spew hatred and ignorance. Thoughtful discussions are derailed by one comment that brings down the intelligence of everyone involved in the thread. Even when you're off the clock, you must resist taking the bait of trolls.
For the uninitiated: internet trolls are people that post comments that are meant specifically to anger another person or are the strong opinions of someone that disagrees with the topic being discussed. More often than not they are the former. They are people that use the anonymity of the internet to the extreme and spew whatever they feel like, as they feel safe that they will suffer no consequences. This is clearly not the case any more. Hackers that are determined can track the ISP of a commenter and identify them quickly. It is highly unlikely that a comment on a public forum will garner much backlash outside of angry comments in response, but the layer of being anonymous isn't secure.
Freelancing carries heavy risks that you might not consider when first starting out. Every blog post you respond to has your named tied to it. Being as professional as possible is the only safe way to post anything when your name is associated with your comment. If your business is your name, you have to be on high alert at all times. Unless you post using an alias, every word you enter is directly tied to your business. Even when you post on your off-time from your job.
As a professional that lives alongside the internet, I normally lean towards the choice of not commenting. Most articles I would comment on are within my field of expertise and don't generate huge threads of debate. They are either comments of agreement or disagreement. How someone thinks or does something doesn't make their way the only way, and sometimes they need to know there are other options and methods. These types of comments are useful and constructive. Others articles or things I could comment on fall into my "not about work" category. These can be teeming with unseen trolls. Comments are normally destructive, highly opinionated, and come across as very close-minded. Not worth the mental anguish of arguing back and forth with an idiot (idiot being my opinion of them, not essentially an accurate representation).
Having strong opinions about things shouldn't prevent you from commenting on topics if you so choose to. Things you say outside of your professional environment will reflect back on you as a professional, even if you didn't mean it as such. Just be mindful that everything you say lives forever on the internet. And don't feed the trolls. They already ate.