If you come to a main or meta site only to downvote or leave negative comments, some people might question your motives. If you really want to help, why are you not upvoting good content or leaving positive comments too? There's a lot of amazing questions and answers here at MMSE that people have spent hours on contributing, so every time you downvote something, why not upvote a few things too?
For the record, I don't recommend to downvote anything on the main site at all (and although I have upvoted more than 5000 posts on the MMSE main site, I've never downvoted a single post), but we don't yet have downvote buttons removed entirely from this site, so until we have them removed we unfortunately have to deal with the fact that some people click them.
The same goes for interacting with individuals. If you find yourself only downvoting or leaving only negative comments on someone's posts, put yourself in their shoes and ask yourself how you would feel if this was happening to you?
Some brand new users do not know much about "Stack Exchange etiquette" and you might find yourself leaving negative comments on their posts more often than usual. First of all, for brand new users, it's quite common to leave positive welcome messages, so I do not feel that 100% of your comments should ever be negative. However, after the welcoming is over, if you find yourself leaving five negative comments in a row (whether on the same question or across multiple questions, and whether on the same site or across multiple main/meta sites) to the same user, perhaps it is time to take a break because the likelihood of making a positive impact with each successive negative comment is drastically decreasing, and the need for such negativity without any positivity is so rare that I worry you might be unfairly harming the site or the individual if you go beyond this amount.
In the case of users that have, say 5000 rep on the main site or 50,000 rep across all sites, such users tend to have been around for long enough to know what they're doing and I recommend a limit of three negative interactions in a row instead of five (remember that you can always balance negativity with positivity by showing appreciation for some of their other positive contributions; there must be at least one of them if they have 5000+ rep! you can also flag for a diamond moderator's attention if you think that someone has written spam more than five times in a row, for example).
What happens if you see targeted negativity beyond the limits suggested in the above paragraphs? I don't think that suspensions or even private messages are necessary the first time that a user does this, but to maintain a positive atmosphere and to help prevent fights or bottled up anger towards the site/community or towards users, I believe that something has to be done. I suggest that if you notice something like this happening to another user's posts, for example five negative comments in a row from the same user, then to just send them a very simple comment with a link to this Meta post for example, or to flag for a diamond moderator to do it if it's you that's being targeted rather than someone else. A comment like: "@user thanks for your efforts to help the user. Following the suggestion [here], let's now allow others provide feedback if necessary". If more negative feedback is really necessary (this will be extremely rare by the way!) then surely it can be done by others (we have plenty of active users here!), so that it doesn't look like anyone is being "targeted" by someone. When you see five targeted negative comments in a row, often some of them will be unnecessarily unkind so I suggest flagging such comments for being "unfriendly or unkind".
