The victim of a crime, like if someone stole from him, cheated him, or even just cursed him out, embarrassed him etc. cannot tell others of what happened to him even If he tries meeting the 7 prerequisites discussed at the beginning of the chapter because there is no way he can tell anyone what happened purely for the sake of the truth to teach people the severity of the crime or to apply pressure to convince the perpetrator repent, he can’t honestly do this because he has personal interests. If people accept what he says he’ll feel justified to tell others and blaspheme the perpetrator and feel good about it since he did a crime against him. There is an exception to this rule that will be discussed in halacha 13.
The Chofetz Chaim feels a need to go into much detail to explain why personal interests are a reason not to speak out since many people make the excuse that they have the right to speak up against criminals who have wronged them personally. Quoting Rabbeinu Yonah he points out that a single witness is only allowed to testify in court if it will be productive, for example it will force the litigant to swear so that the truth will hopefully come out. But one who testifies about what happened to himself in court cannot force and oath because he is invalidated to be a witness for himself, if he testifies in court it is lashon hara and there is a separate sin for the judges to accept what he said, called “lo sisa shema shav” this prohibition will apply if he spoke to the judge before the court case about what happened, even what he says is true, and certainly if he just tells anyone else what happened to him it is lashon hara and anyone who accepts it even if it is true is accepting lashon hara which is forbidden. The only exception would be if a messenger of the court was sent to bring someone to court and he was cursed out and got a refusal to come to court, for example when Nadav and Avihu refused to show up before Moshe then the messenger of the court is allowed to tell them he was cursed out, as mentioned in Moed Katan 16a. The Ram”a in Shulchan Aruch in Choshen Mishpat 228:1 poskins that if someone insults you, you can insult him back halachically (though it is good character to stay silent) but that is only in private, that does not mean you can go reveal to others that someone insulted you.