The name of the slain Israelite man who was slain with the Midianite woman was Zimri son of Salu, leader of a father's house Simeonites. (Bamidbar 25:14)
At the end of Parashat Balak the Torah relates that a Jew, in a shocking act of brazenness, brought a Midianite woman to Moshe and sinned with her in public view. Pinehas seized a spear and killed both the man and woman. In the beginning of Perashat Pinehas the Torah proceeds to reveal the identity of that man: Zimri the son of Salu. Why does the Torah wait until now to reveal to us who this man was?
When a person sees someone else sin, his sensitivities weaken and it is easier for him to sin as well. This is even more the case when the person who sins is regarded highly in society. Zimri held a high position; he was the leader of his tribe and one of the great people of the generation. Had the Torah mentioned at the time of the sin that the person sinning was Zimri, it would have had a tremendous negative influence on the people. People would reason, "If such a great person can do something like that, then surely I can!" Therefore the Torah waited until Pinehas killed him and demonstrated that such a sin is not tolerated no matter who you are. Now revealing the sinner would not have any harmful impact.
This demonstrates the magnitude a person's actions can have, affecting others positively or negatively. People watch what you do and follow suit. Even better than reproving others is to rectify one's own actions. If you have a big cup with a bunch of little cups surrounding it, there are two ways to fill up the cups. Either pour into each cup individually, or pour into the big cup, allowing it to overflow into all the small cups. When a person is overflowing with kindness, everyone around him becomes kinder.
The Hafetz Haim used to say that in his younger years he wanted to perfect the world. That didn't work so he figured he would work to perfect the country in which he lived. That also didn't work so he decided that he would just perfect his home town, which again failed. He finally decided that he would just try to perfect himself. By perfecting himself he then influenced his home town, the whole country and Jews worldwide!
If other people's negative actions disturb you, you do not have to reprove them constantly. For example, if people come late to shul or are not sensitive with their speech, you can be extra careful in those areas and this will cause a change in them. Be the change that you want to see in the world!
Rabbi Isaac Farhi