Rashi to 25:11
Rashi says that the Torah traces the lineage of Pinchas back to Aharon Hakohen, because the other tribes were humiliating him saying “Did you see the son of Puti, whose grandfather fattened calves for idolatry, yet he killed the prince of a tribe of Israel?”
Rashi tells us that after the act that Pinchas did, there were those who felt it was improper for him to have done so. Therefore, the Torah traces his lineage back to Aharon Hakohen, who was known by all to be without blemish, in order to show everyone it was proper for him to act as he did. Rav Yitzchak Isaac Sher quotes a medrash in last week’s parsha that goes into detail listing 12 miracles that were performed by Hashem (or a malach) in order to have the act of Zimri and Cozbi clearly understood as the chilul hashem that it was. He then asks: “If Klal Yisrael saw such miracles, it must be clear that this is what Hashem wanted to be done, so how could they justify speaking negatively about Pinchas at this time?”
He answers his question with a very important concept of which people should be aware: An act which should be done out of religious outrage, even though the act is correct, still must only be performed by a person with sterling character. If the zealot is blemished, even in the slightest, though his intentions are pure, this does not necessarily absolve him from being guilty of killing a prince of Israel. This is because if the source isn’t pure, it is impossible for the act to be 100% pure, despite the pure intentions. (According to American law, when an item is labeled “pure” it does not mean that it only has the main ingredient. In fact, the FDA permits up to 2% of other additives (this is very significant in regards to kashrus, in which “batel b’shishim”is only 1.67%). However, “100% Honey” or “100% Maple Syrup” does indeed mean that the product must contain 100% of said ingredient.)
In our avodas Hashem, it is generally acceptable for our devotion to be 98% “pure”. However, something which falls under the banner of ‘kanoas’ must be 100%.
In a similar vein, many times, we see our neighbors and friends doing something which we would like to correct. We must check ourselves to make sure that we are on the level to be the one to call their attention to their wrongdoing. If we feel we are not, it is our duty to find the correct person who will be able to create a receptive ear in the person who needs the rebuke.
I would like to offer another answer to the question. People believe what they want to believe. They emphasize certain facts and diminish others in order to create a picture which makes them comfortable. As the three weeks are upon us, and some of us already have begun to learn the story of the destruction of the first and second temples in Jerusalem, we see that there were reputable prophets chastising klal Yisrael, and trying to put them on the proper path, yet people were always finding excuses to hear and perceive events as they pleased. Possibly, in tongue and cheek, we could say living in the exile is living in the reality which we create, as a way of trying to avoid facing the dismal reality that we are living in Galus, meaning: Not the way the world is supposed to be. Even though we should be aware of this the whole year, it is this time of year which was set aside for us to look in the mirror and see our lowly state and cry out to Hashem that he should better his lot and ours, by rebuilding the Beis Hamikdash quickly.