Assalaamu alaykum. I sometimes speak in a very angry voice. I sometimes like to do so as it clears my mind. When I supplicate, I do so as well sometimes. I am not angry with Allaah; sometimes I am just angry about some stuff, if you speak with an angry voice to Allaah but are not angry with Allaah or are not menacing to show disrespect. Is that kufr (disbelief) then, or just a sin?

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 ʻalayhi wa sallam, is His slave and Messenger. 

It is disliked to raise one's voice while supplicating, especially if it is intended as an expression of anger. In principle, one should recite thikr (remembrance of Allaah) and supplications inaudibly; Allaah, The Exalted, says (what means): {Call upon your Lord in humility and privately; indeed, He does not like transgressors.} [Quran 7:55] Moreover, Abu Moosa said, “We were with the Prophet, sallallaahu ʻalayhi wa sallam, on a journey, and whenever we ascended a high place, we used to say (loudly), 'Allaahu Akbar'. The Prophet, sallallaahu ʻalayhi wa sallam, said, 'O people, take it easy upon yourselves! You are not calling upon someone deaf or absent. You are calling upon One Who is All-Hearing and All-Seeing.'” [Al-Bukhari and Muslim]

Hence, many scholars asserted that it is recommended to recite supplications and words of remembrance of Allaah inaudibly, without raising one's voice. Shaykhul-Islaam Ibn Taymiyyah wrote:

“The Sunnah of the Prophet, sallallaahu ʻalayhi wa sallam, in this regard is to lower one's voice while imploring Allaah and recite supplications inaudibly except for a valid reason as per the sharee'ah. Allaah, The Exalted, says (what means): {Call upon your Lord in humility and privately; indeed, He does not like transgressors.} [Quran 7:55] He says about Prophet Zakariyya (Zechariah) (what means): {When he called to his Lord a private supplication.} [Quran 19:3] He also says (what means): {And remember your Lord within yourself in humility and in fear without being apparent in speech - in the mornings and the evenings. And do not be among the heedless.} [Quran 7:205] Moreover, it has been narrated that the Companions were with the Prophet, sallallaahu ʻalayhi wa sallam, on a journey and they raised their voices while reciting supplications. He, sallallaahu ʻalayhi wa sallam, said, 'O people! Be merciful to yourselves (i.e. do not raise your voice), for you are not calling a deaf or an absent one but One Who is All-Hearer and Ever Near (to all things). He Whom you are supplicating is nearer to every one of you than the neck of his camel.' [Al-Bukhari and Muslim]” [End of quote]

At-Tabari commented on the verse that reads (what means): {And remember your Lord within yourself in humility and in fear} [Quran 7:205] saying, “The verse means that the Muslim is enjoined to display total submission to Allaah while imploring Him and that it is disliked to raise one's voice or shout while reciting supplications and calling upon Allaah.” [End of quote]

For more benefit, please refer to fatwa 90031.

However, it is not considered kufr; it is merely disliked, as was stated by Ibn Jurayj and others. If a wronged person was enraged at the one who wronged him and supplicated Allaah against him in a state of anger, then this is neither kufr nor disrespect of Allaah on his part.

Allaah knows best.