All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.
The aggressive behavior you have described is included under causing harm to a Muslim and scorning and mocking, which are forbidden in Islam, and those who do it are dispraised. Allaah says (what means): {And those who harm believing men and believing women for [something] other than what they have earned have certainly born upon themselves a slander and manifest sin.} [Quran 33:58]
Ash-Shawkaani said in Fat-h Al-Qadeer in his interpretation of the above verse: "It means those who harm believing men and believing women with any kind of harm, either by words or actions, and the meaning of {for [something] other than what they have earned} is that this is not due to something which they have done and which requires harming them and which they deserve…{have certainly born upon themselves a slander and manifest sin.} meaning it is clear and obvious that it is a slander and a sin." [End of quote]
Allaah also says (what means): {And do not insult one another} [Quran 49:11] As-Sa‘di said in his interpretation of this verse: "That is, they should not slander one another. The term Lamz used in the verse means insulting by words, and the term Hamz means insulting by action, and both are forbidden and there is a threat of the Fire for them." [End of quote]
As-Sa‘di said in his interpretation of the verse in which Allaah says (what means): {Woe to every scorner and mocker}, {Woe} i.e. warning, bad consequence and severe punishment, {to every scorner and mocker}, who scorns the people with his actions (Hamz) and mocks them with his words (Lamz); so the scorner is the one who scorns people and insults them by gestures and actions, and the mocker is the one who insults them with his words." [End of quote]
As regards whether or not there is a Hadeeth about accusing a Muslim with something he is innocent of being a sin heavier than the weight of the sky, we are not aware of a Hadeeth related to this, and we have not found it. However, the Prophet, sallallaahu ‘alayhi wa sallam, said: "Whoever says something about a believer which is not true about him, Allaah will house him in the pus of the people of the Fire until he takes back what he has said and he will not be able to do so." [Abu Daawood - Al-Albaani graded it Saheeh (sound)]
Furthermore, the Prophet, sallallaahu ‘alayhi wa sallam, said: "Whoever protects a believer from a hypocrite" (I think he said) "Allaah will send forth an angel who protects his flesh on the Day of Judgment from the Fire of Hell, and whoever accuses a Muslim of anything, desiring to disgrace him, Allaah will hold him over the bridge of the Fire until he takes back what he has said." [Abu Daawood - Al-Albaani graded it Hasan (good)]
As regards how Allaah will exact retribution against the scorner and mocker for the person who is wronged, this is a matter of the Unseen that only Allaah knows, and we are not aware of a Hadeeth about the how those who scorn and mock people will be punished on the Day of Judgment.
As regards whether or not Allaah will accept their repentance if they repent, then the answer is, yes – Allaah accepts the repentance of a person from any sin, as Allaah says (what means): {Say, "O My servants who have transgressed against themselves [by sinning], do not despair of the mercy of Allaah. Indeed, Allaah forgives all sins. Indeed, it is He who is the Forgiving, the Merciful."} [Quran 39:53] Ibn Katheer said in his interpretation of this verse: "This is inclusive of all kinds of sins, disbelief, idolatry, doubt and hypocrisy, killing, dissoluteness, and other sins: anyone who repents from any of such sins, Allaah will accept his repentance." [End of quote]
He also said: "This distinguished verse is a call to every sinner, whether a disbeliever or anyone else, to repent and return to Allaah, and it provides that Allaah forgives all the sins of anyone who repents from them and stops doing them no matter how grave or numerous, even if they were like the foam of the sea."
Nonetheless, this does not mean that the rights of the person who is harmed with slander or injustice will be lost; rather, Allaah will grant him his right or He will satisfy him on the Day of Judgment. 'Umdat Al-Qaari Sharh Saheeh Al-Bukhari authored by Badr Ad-Deen Al-‘Ayni Al-Hanafi says about the interpretation of the Hadeeth about the one who killed 99 people and then died after having repented and that Allaah forgave him: "His words, 'so he was forgiven' means that Allaah forgave him. If it is said that the rights of humans are not forfeited by repentance and in fact satisfaction should be sought, it will be answered that when Allaah accepts the repentance of His slave, He satisfies the person who contends in a plea against him."
Furthermore, Ibn Hajar commented on this Hadeeth saying: "It can be interpreted that when Allaah accepts the repentance of the killer, He undertakes to satisfy the one who contends in a plea against him."
Finally, we advise you to be patient with people's harm as Allaah will reward you for being patient. We also advise you to take account of yourself as there may be something about you which you do not know about which has led them to harm you.
For more benefit on the prohibition of mocking, please refer to Fatwa 86381 and refer to Fatwa 83577 on the wisdom behind affliction and the virtue of being patient when afflicted.
Allaah Knows best.