I've seen this a few times during the past few years. Most of the times they were only subtly derogatory comments, but on rare occasions openly bellitling. Whether the comments were well-deserved or not is not the point here. My question is: is it unethical to post criticism on somebody else's answer when you have also posted an answer yourself?
It hasn't happened to me and I'm not taking sides here.
'Unethical' seems to be too strong a word for it. I find it a slighty dodgy depending on the circumstances.
As others have said, whoever said it, if the comment is questionable, it is questionable. If another answerer said it, there is extra added on because of the possibility of conflict of interest. Even though rep points is entirely fictitious, it can feel like betting or a race or [pick your own 'winning' metaphor]. And a comment on another answer that attempts to bring the score down (however rational that might be) seems like it's being done in one's own favor.
If everything were purely rational, say questions like addition, one answerer who believes themselves correct might want to point out a supposed error in another's different answer (to help explicate an error). But it still seems dicey. When things are vaguer, such comments can feel childish.
I would not go so far as to say unethical but merely questionable, uncool, or self-serving, but it depends on the context.
Let's be clear about what we're taking about. "Derogatory" or belittling comments are never OK, whether you've posted an answer or not. The "Be nice" policy applies to everybody, in all situations. If you think a comment crosses the line, flag it.
Keep in mind that not all criticism crosses this line. It is not unethical to post civil and constructive criticism. That's one of the main purposes for comments.
So let me rephrase the question to avoid the irrelevant parts. I'll assume by "negative comment" you mean a comment that you would be fine with if it were posted by someone who hadn't written an answer.
Why would it be? Someone who knows enough to be able to answer the question is more likely to know enough to criticize "competing" answers.
I put "competing" in quotation marks because competition is really not the point of this site. The point is to give the OP and future visitors the best answer or set of answers possible.
The only concern that I can see is the possibility that people might try to abuse the system and try to make their answers more prominent by posting false or harmful comments in an attempt to get other answers downvoted or deleted. And this is really not very likely. People's names are attached to their comments. As I said earlier, if somebody posts a bullying, belittling comment, there are many other users who will see it and be able to react: they can flag it to get a moderator to deal with it, or post a comment in response that explains how it is false.
In fact, even though it's much more prone to this type of abuse, people can even downvote other people's answers! Downvotes are anonymous and actually affect the ranking of the answers. But Stack Exchange still allows it. Some of the reasons are outlined here: Does answering a question lock you out of downvoting on other answers to that question?
As Jon Skeet says:
No-one benefits from a correct answer being hard to distinguish from an incorrect one because you wanted to be sportsmanlike :)
I regularly post critical comments on answers that I think could be better, even if I've also posted an answer to the question. Obviously, I try not to be rude or belittling. I don't know how many might be considered "subt[ly] derogatory": that's a pretty vague phrase, and it seems it would be easy to either miss this kind of "subtle" rudeness, or to see it where no rudeness was intended. It's certainly not the tone I aim for.
I also post positive comments on other answers if I think the answer is good. Same goes for voting: I'll downvote an answer if I think it's bad, and upvote it if I think it's good.
In my view there is no correlation to the fact if I have posted an answer as well. All comments should be civil and constructive and no comments that are belittling or derogative are ever okay.
If I answer a popular question the review queues will regularly prompt first answers and low quality answers of a question I answered too. Therefore the review system actively encourages me to react to answers - in whichever way. On the other hand the badge system offers a badge if I up vote answers of questions where I also answered.
Anyways one should always react to an answer based on its own merit - or lack thereof, not based on the fact if oneself has answered, too.
