The Chofetz Chaim says there are many details defining what being concerned over lashon hara might mean which will be discussed in more detail in the last chapter, but the general rule is that you can take proper precautions to defend yourself or others but under no means are you allowed to take any actions against him, whether physically hurt the potential threat or disgrace and embarrass him in any way as long as it is unclarified.
The Chofetz Chaim brings down a responsa from the Mahari”k (chapter 188) which is a live illustration of going beyond being concerned. There a story a poor old Jew named Reb Aharon Ruskia who a woman spread rumors that he was adulterous with her and people ran him out of town by publicly embarrassing him and not even allowing him to get an aliyah in shul. When the Mahari”k heard about this he was furious and said it’s a great sin to believe this cursed woman! A person who embarrasses and denigrates a descendant of Avraham, Yitzchak, and Yaakov, treating him lightly in your eyes, is considered disgusting in the “eyes” of Hashem and will be greatly punished! There is no worse embarrassment then what was done to Reb Aharon, the way he was treated by not giving him an aliyah. Furthermore, the Gemara in Bava Metzia 58b proves from the episode of King Dovid and Batsheva, when Doeg and Achitofel tried talking out and embarrassing King Dovid in public, that embarrassing someone in public is worse than adultery. For King Dovid said, even if I was adulterous (which he wasn’t) then that deserves the capital punishment of strangulation but still he would a get a share in the World to Come but one who embarrasses another in public has no share in the World to Come (if he doesn’t repent before he dies.) Therefore, you have to be very careful and thoroughly check out the matter before you come to conclusions that a person did a wrongdoing which deserves such severe punishment like public excommunication.
Being concerned for what you heard does not mean to be on the offensive and taking action against the would-be perpetrator, it only means to be on the defensive. One must be very carefully when telling others of a possible threat to be sure they will just listen and take precautions on the defensive because if you see they will go on the offensive then you cannot warn then since the whole reason you are allowed to warn then is because of the verse “Love your neighbor as yourself” just as you would not want any harm done to yourself then you should inform others who might be in harm’s way. However, if you know the people you will tell will go on the offensive and hurt or embarrass the would-be offender then why should his blood be any redder than their blood and the mitzvah of “Love your neighbor as yourself” will apply to the possible offender because he might be a threat to others, but they are definitely going to be a threat to him so it’s better. It to tell them anything.
Bottom line you can only be concerned about lashon hara you heard to be on the defensive but not to act on the offensive.