In my previous posts, I have laid out the nature of harm in some detail. Harm is acting contrary to life. But since life includes those powers and abilities that exist because of our humanity, preventing their exercise is an example of harm. Harmful speech is not a specific kind of speech because all wrong speech is harmful and harmful speech has no characteristic shared in virtue of being harmful.
Harmful speech that is not lying, deceiving, promise-breaking or gossip includes such things as making a record of past wrongs and pointing them out, pointing out characters flaws for malicious reasons and pointing out that someone’s future dreams are unlikely to materialize. The problem is that any element of speech insofar as it is evil is harmful. Gossip breaks trust, lying and deceiving harms belief and trust and promise-breaking also harms trust. This does not count the results of these harms or the harms on others. So harmful speech is not really a distinct kind of speech.
In fact, harmful speech is not a kind of speech at all. It is simply a characteristic of evil action in general. All evil actions harm someone, even if the only person harmed is the person doing the evil action. But deception, lying, gossip and promise-breaking all require the use of signs in order to be possible at all. It is not possible to lie without communicating. Therefore, those kinds of actions are distinct ways that speech, insofar as it is a form of communication, can go wrong. But harmful action is not like that. One can harm in any way at all.
Harmful speech is simply another way of harming someone. It is wrong for the same reasons that harm is wrong. It can be understood completely by understanding harm in general. There is nothing about speech in particular that is required in order to understand harm.
This means that there are really only four ways that speech may go wrong. It may go wrong with respect to the truth. Lying and deception are (at least sometimes) wrong because they lead someone to a false opinion. It may go wrong with respect to trust. Gossip breaks the trust one places in another person by giving them sensitive information. In nations, such a breaking of trust is called treason. Finally, it may be reckless. This accounts for the breaking of promises that does not fall within the other two categories as well.
Since recklessness and breaking trust can both happen without communication, one might claim that harm is similar to them. The difference is that the specific kind of wrongdoing found in speech in those cases is necessarily limited to speech. It is not possible to engage in promise-breaking without making promises, and gossip requires communication. But harmful speech is not a specific kind of speech. Sometimes harmful speech can divide people, but sometimes it can bring people together by degrading other people. So there is no one characteristic one that all harmful speech shares in virtue of being harmful.