It is a law of “loshon hora physics” that when one speaks loshon hora about the spiritual failings of someone else, that loshon hora is most intense and righteously indignant.
Unfortunately, to many people there is nothing more self-satisfying than identifying and disapproving of someone else’s deficiency; e.g., that someone does not help his parents or learn with his children, or does not do some mitzvah that the speaker happens to observe carefully.
The Chofetz Chaim informs us that it is loshon hora to say that a person has transgressed a positive or negative commandment, whether the mitzvah observance is one generally performed carefully, or one that is largely overlooked. Even if the criticism is only that the person does not do the mitzvah in the optimum manner — for example, he does not spend as much as he should on items for Shabbos — it is forbidden to relate it.
Obviously there are times when mention of someone’s laxity in mitzvah observance might be necessary. At times, one needs to warn a child to stay away from someone who is a bad influence. In such cases, it is worthwhile to ask a posek (halachic authority) how to relate the information in a way that is permitted by halachah and does not create unnecessary harm.
R’ Shamshon Raphael Hirsch once commented on the common urge to speak loshon hora about a fellow Jew’s laxity in mitzvah observance. He said that the soul’s natural tendency is to strive ever higher. If a person is actively involved in Torah and mitzvos, then he is growing spiritually and his soul is content. But if a person is lazy and his actions are not helping his soul to move upward, then he feels inner discontent. He seeks to satisfy this discontent by appearing to be growing spiritually. And how does he accomplish this? By making everyone around him appear smaller. His thinking goes something like this: “If my fellow Jew doesn’t give enough tzedakah (charity) or do some other mitzvah that I am careful to do, then by focusing on his deficiencies, I will feel as if I am higher.”
This type of loshon hora works much like a drug for the soul. When the person makes use of it, he feels righteous and holy. But as soon as its effect wears off, he realizes that he is no higher than before. If anything, he is lower.
The Torah does not want us to find fault with our fellow Jews’ mitzvah observance. When we denigrate Jews, we not only do something lowly, but we also lull ourselves into a false sense of complacency. Nothing good comes from fooling ourselves, from being content with a false sense of spiritual achievement. Hashem wants us to strive for holiness in our lives, to make spiritual gains which are real and meaningful. The way to do this is by viewing ourselves in an honest, critical way, while seeing others in a positive light.