The Chofetz Chaim concluded halacha 2 saying that one can also not speak lashon hara about one who is lacks in even branches of a mitzvah, meaning he keeps part of a mitzvah, for example if a person is frugal with his money and does not honor the Shabbos to the best of his ability like if he can wear nicer clothes for Shabbos or more delicacies at his Shabbos table and just doesn’t do so because he’s too cheap to buy them. You can’t tell anyone this even if you have firsthand knowledge. Also if one is lacks in rabbinic mitzvahs, whether they are fences to stop one from sinning like muktza or decrees like lighting the menora on Chanukah or even if they are seemingly minor rabbinic enactments like how to tie one’s shoe or how to cut your nails, which are both discussed in the Shulchan Aruch, it still constitutes lashon hara to tell others about his wrong doing. However if he doesn’t care about the rabbis and has an attitude that there is no need to keep rabbinic mitzvos then this is a different situation which Rava refers to these type of people as sinners in Shabbos 40a and one who is not only not holy but also an evil person, in Yevamos 20a, in that case there is a mitzvah to speak out against him.
Halacha 3 reiterated that there are a lot of details that must be taken into account when seeing someone do something that he is not supposed to be doing halachically. If he is the average Jew like most of us are who usually is careful about sinning but sometimes transgresses then there is a mitzvah to judge him favorably that he did it unintentionally or didn’t know it is forbidden, or just thought it was a restriction, or nice thing to do which only the righteous are careful in and therefore it is forbidden to tell anyone the sin you saw or heard someone else transgress, even if two people saw it. You can’t even hate him because there is a mitzvah to judge him favorably. There are different halachos in terms of speaking lashon hara and having that person allowed to be a witness. But one has to judge him favorably even if it is a sin people know is a problem, or even assume it was intentional though many people are lacks in the matter, even if the person transgresses it many times. However if he is assumed to be doing a certain sin all the time then the Halacha might change and will be discussed later in halacha 7.