Let's imagine a situation:
Mr. X posted a question in Math SE. He's a beginner, and the question may seem naive for many of us. Let's suppose his question still meets the rules of MSE.
Now, Mr. Y reads the question, laughs out loud, and writes in comments:
Your question is stupid, dude.
(of course, Mr. Y is not a beginner and writes the sentence above in such way that his comment won't be flagged).
The question of Mr. X is then sawn by many individuals. Some of them are encouraging, try to find easy ways to explain the problems. Others, (I think about Mr. $\mathcal{X}$, Mr. $\mathcal{Y}$ and Mr. $\mathcal{Z}$), are less benevolent and upvote the comment of Mr. Y.
What will be the first thing Mr. X sees when he logs in in MSE the next time ?
(+3) Your question is stupid, dude.
Even if other commenters have been kind to him, or even reproach Mr. Y his unkind comment, the impression of his upvoted message won't be negligeable.
What I would like to be able to do, is to downvote some wrong/unkind comments. There are many reason why we can dowvote questions/answers. Souldn't we be able to do the same with comments ? Why can we only upvote them ? It is impossible to see if a reaction is biased if you can only express your agreement. And everybody won't take the time to write a comment contradicting the first one. What do you think about this ?