I wanted to give you a proper response to this but ran out of time. In any case, thanks for the kind reply!
If you felt like the comment was breaking the site guidelines (for example against shallow dismissals), then yes, a flag would be appropriate; or perhaps a downvote as a slightly lesser expression.
In terms of "people like this": the trouble is that we can't tell much about each other from these tiny globs of internet text, and what ends up happening is that we connect the dots in ways that come from our own prior experiences (usually negative ones), and then we end up feeling surrounded by jerks.
The root issue is not that everyone is that big of a jerk (though some do behave that way)—it's that we don't have enough information. Shallow comments expressing negative opinions are the most likely to generate this feeling in the reader, because the combination of lack-of-information and negative-sentiment is sort of fatal.
From a moderation point of view: if you read enough of these threads, it turns out that every reader has a different set of triggers for what feels to them like "people like this" whose "purpose is to negate" and so on. People thus have completely different interpretations of which are the worst comments. This makes sense once you understand that we're all filling in the blanks, connecting the dots, etc. out of our own prior experience. We all have such different priors that we will in different blanks in different ways.
This is not a basis for strong vocal moderation because if I scold one user saying "you're being purposely negative, please don't", what happens is a great many readers for whom the comment didn't land that way will feel like the mods are being heavy-handed and forcefully protest. Moreover, the user being scolded will most often reply, quite sincerely, that they had no intention of being that way and had no idea that their comment could land that way.
When running into a comment like that which creates a feeling like that, the temptation is to reply forcefully—i.e. with as much frustration as the comment generated in you—but for the purposes of a good online discussion, this is the worst thing to do. It only creates even stronger negative feelings in others, who then feel entitled to strike back in turn—and we get a downward spiral which is equal parts tedious and nasty.
The only thing I know that works is to "metabolize" your negative reaction by letting it run its course in you, and eventually subside, before replying. At that point, you have two options available that you didn't have before: either to reply neutrally, or to simply move on to something else that's more interesting.
Of course this advice is not so easy to follow in practice, and what it requires of you (I don't mean you personally of course, but all of us) is much deeper than it appears. On the bright side, it does mean that internet forum communication is not quite as trivial as it appears!