A person mourning the death of one of his parents says Kaddish during the first year. Saying Kaddish is of great value to the deceased; it saves him from the judgment of Gehinnom, for since his son, whom he left behind in this world, is sanctifying God by reciting this prayer, his merits increase. One must say Kaddish even for completely righteous parents, as well as for someone who was killed for the sanctification of God’s Name. Concerning righteous parents, though they are already promised life in the next world, their sons’ Kaddish elevates their souls and causes them to be at peace. Certainly, one must recite Kaddish for evil parents since they are in great need of Kaddish which diminishes the judgment of Gehinnom. We even say Kaddish for someone who committed suicide. Although the Chachamim teach that we do not mourn for him and he does not have a share in the next life, the Kaddish that his sons recite for him helps repair his soul slightly.
If the son knows how to lead the services, he should be the chazan on weekdays, since this benefits the deceased even more. If it is difficult for him, he should try to lead the prayer service from Ashrei after Tachanun until the end of the service. It is especially good for him to be the chazan for Ma’ariv, since at that time the judgments from Gehinnom intensify. This is particularly true on Motza’ei Shabbat, when the souls return to Gehinnom after their respite during Shabbat. It is important to note that according to the Ashkenazic minhag, on Shabbat and festivals, those in mourning say Kaddish but do not lead the services (Rama, Yoreh De’ah 376:4). Even on Rosh Chodesh and Chanukah, it is customary that the mourners do not lead the services.
This only applies when the son knows how to be the chazan, but if he does not know how to say the words properly, or his voice is unusual, or he excessively shortens or extends his prayer, it is preferable for him not to be the chazan. Rather, he should suffice with saying Kaddish. If he insists on being chazan against the wishes of the congregation, instead of sanctifying God’s Name, he desecrates the Name of Heaven and causes pain to the deceased. In general, mourners must know that despite the importance of serving as the chazan and the recital of Kaddish, the most meaningful and beneficial acts children can do in order to aid the ascent of their dead parents’ souls are to increase their Torah learning, their giving of charity, and their fulfillment of other mitzvot.
There were certain communities in which, over time, more and more people did not know how to lead the prayer service, and therefore they established that mourners should not lead the services. However, for Ma’ariv, it is proper not to prevent mourners who are capable of being chazanim from leading the prayer service (Mishnah Berurah 53:61).
.In Masechet Sofrim, chapter 19, it is written that people in mourning say Kaddish. Or Zarua, part 2, end of section 50, brings a story about a dead person who suffered greatly because of the sins he committed while he was alive. Rabbi Akiva saw him and wanted to save him from his suffering. He found out that he had a son who was completely uneducated, so Rabbi Akiva put great effort into teaching him until he could say Kaddish for his father. Afterwards, the dead person appeared to Rabbi Akiva and said to him, “Rabbi, let your mind be at ease in the Garden of Eden just like you put my mind to rest and saved me from the judgment of Gehinnom.” Here we see that the Kaddish indeed helps evil people (Beit Yosef, Yoreh De’ah 376). The Chatam Sofer (Even HaEzer, end of section 69), writes that a person who commits suicide does not have a share in the World to Come, but can be saved from the judgment of Gehinnom through prayer, in the same manner that David prayed for his son Avshalom (See Yabia Omer part 6, Yoreh De’ah 36). On a parallel but distinct note, the Maharil (responsa 96) writes that Kaddish is said for righteous people as well.
One who needs to say Kaddish but arrives late to pray: if the congregation finished the Shir Shel Yom (Psalm of the Day) while he is still in the middle of saying Pesukei d’Zimrah, he may interrupt to recite Mourner’s Kaddish, if there will not be another chance to say it. However, for Kaddish d’Rabbanan one may not interrupt (Maharshag 1:48; Yabia Omer, part 7, 10).