The gemara is trying to find a case where there is a תיקון in the process of tearing and destroying keilim. Even according to R. Yehuda who says that one is chayev for a מלאכה שאינה צריכה לגופה, it must be a tikun and not a kilkul. The gemara suggests that we are speaking about a case where one tears something to allow his anger to subside. Since there is a tikun for him, it qualifies as a tikun despite that it is a kilkul to the item (this only works in r. yehuda who expands the chiyuv to even מלאכה שא"צ לגופה, therefore we can also expand the scope of what we consider a tikun, just as we do when one causes an injury because they need the blood - 106a). The gemara then asks, is this permitted? One is not allowed to let out their anger for the purpose of letting off steam because he is training his yetzer hora to behave in this very aggressive manner. Rashi explains that we don't consider letting off steam to be a tikun since it encourages this type of behavior to continue. According to Rashi, the Torah demands that we control our aggression, and only through suppressing it will one overcome it. Therefore, the only tikun in tearing clothing is when one needs to fake anger to instill fear on his children.
The Rambam in 3 different places, seems to say that it is considered a tikun to let off steam by breaking and damaging things. The Rambam writes (Shabbos 8,8) that one who damages someone to let off steam and calm himself down is considered a tikun and is chayev. Similarly (10,10) when tearing clothing out of anger, the Rambam considers it a tikun. And finally (12,1) torching someone's home out of anger he also considers a tikun. The Rambam seems to be inconsistent with the gemara because the gemara implies that we don't regard this kind of behavior as a tikun. The Magid Mishna (8,8) explains that the gemara never meant to challenge whether this is considered a tikun. Although it is considered a tikun, the gemara asks whether it is permitted to act in this way even during the week and proves that it is forbidden. From the fact that the Mishna never discussed the prohibition during the week, we are compelled to find another situation so that the prohibition is only applicable to shabbos and not during the week. Nevertheless, the rule of the gemara that when one damages something to let out their anger, it qualifies as a tikun, remains true. This approach fits with the language of the gemara וכי האי גוונא מי שרי וגו, because the gemara is asking whether this behavior is permitted even during the week. But, according to Rabbeinu Chananel's girsa וכי האי גוונה מי מיחייב, we are forced to interpret like Rashi (not like the Rambam) that the gemara is proving that this is destructive behavior, not a tikun, and therefore one wouldn't be chayev on shabbos for doing this.