Someone once told me that he heard his rosh yeshiva say, "The most dangerous person in the world is someone on his way to do a mitzvah." For some, nothing matters but for them to do their mitzvah, no matter what.
One of his disciples had invited him for Friday night dinner. R. Israel[Salanter] had stipulated that he would not dine anywhere till he had satisfied himself that the kashrut was above reproach. The disciple informed R. Israel that in his home all the Halachot were observed with utmost stringency. He bought his meat from a butcher known for his piety. It was truly "glatt" - free of any Halachic query or lung adhesion (sirchah). His cook was an honest woman, the widow of a Talmid Chacham, daughter of a good family, while his own wife would enter the kitchen periodically to supervise. His Friday night meal was conducted in the grand style. There would be Torah discussion after each course, so there was no possibility of their meal being "as if they had partaken of offerings to idols." They would study Shulchan Aruch regularly, sing zemirot and remain seated at the table till well into the night.
Having listened to this elaborate account of the procedures, R. Israel consented to accept the invitation, but stipulated that the time of the meal be curtailed by two full hours. Having no alternative, the disciple agreed. At the meal, one course followed another without interruption. In less than an hour, the mayim acharonim had been passed around in preparation for the Grace after Meals.
Before proceeding with the Grace, the host turned to R. Israel and asked: "Teach me, Rabbi. What defect did you notice in my table?"
R. Israel did not answer the question. Instead he asked that the widow responsible for the cooking come to the room. He said to her: "Please forgive me, for having inconvenienced you this evening. You were forced to serve one course after another - not as you are used to do."
"Bless you, Rabbi," the woman answered. "Would that you would be a guest here every Friday evening. My master is used to sit at the table till late at night. I am worn out from working all day. My legs can hardly hold me up, so tired do I become. Thanks to you, Rabbi, they hurried this evening, and I am already free to go home and rest."
R. Israel turned to his disciple. "The poor widow's remark is the answer to your question. Indeed your behavior is excellent, but only as long as it does not adversely affect others."1
--The Mussar Movement, Volume I, Part 2 pages 226-228
Sometimes our idea of diligence is at the expense of those around us. We feel we have the obligation to fulfill our religious duties at whatever price to those around us.
As we see from this story regarding Rabbi Salanter, he was very diligent and scrupulous in his religious observance. His attention to detail in serving the Holy One was exemplary. However, he also understood that his status within the community would affect those who followed him. His desire to raise observant but responsible disciples is evident from this story. To serve the Creator without any regard for His creation is not diligence - it is inconsiderate and not a proper reflection of Hashem in our actions. It is not enough to do a mitzvah - it is to do the mitzvah and not harm or take advantage of anyone in the process.
As we strive to increase our diligence in observance and performance of mitzvot, may we do so with proper intent: to lift up Hashem in our midst, and to make sure no one is lowered in the process.
1. Told by R. Dov Rovman in the name of the aged Shachevitz.