What if someone (silently)cursed a disbeliever who mocks Islam and the Prophet, out of anger and later regrets it for being hotheaded is there a chance that the curse may return to them because they are not such strong believers in the first place?

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.

We commend your indignation, and that you take offense when Allaah is disobeyed and are angered at the mockery of those who ridicule Islam and the Prophet, sallallaahu ‘alayhi wa sallam. In fact, such anger is characteristic of the believers. ‘Aa’ishah said, “The Messenger of Allaah, sallallaahu ‘alayhi wa sallam, did not take revenge for himself unless the limits of Allaah were transgressed. Then, he, sallallaahu ‘alayhi wa sallam, took vengeance for it for Allaah.” [Al-Bukhari and Muslim]

Muslim jurists have held different opinions regarding the religious ruling on cursing a specific disbeliever by name. We have previously underlined the different scholarly opinions and the most evident opinion in this regard in Fatwa 56543, so please refer to it.

The curse returns to the curser only when someone curses a person or thing undeservingly (i.e. he or it does not deserve to be cursed). Ibn ‘Abbaas reported that a man cursed the wind. The Prophet, sallallaahu ‘alayhi wa sallam, said: “Do not curse the wind because it is commanded [by Allaah to blow]. Whoever curses something and it does not deserve the curse, the curse will turn back upon him.” [Abu Daawood]

Moreover, Allaah, The Exalted, has cursed the disbelievers in general. He says (what means): {Indeed, Allaah has cursed the disbelievers and prepared for them a Blaze.} [Quran 33:64]

Allaah Knows best.