I was angry at my family for some personal stuff going on and I talked bad about them with someone else. They don’t know but I said some horrible stuff like how they can die and go to hell. I apologized to them for being rude that day but I still feel bad for the other things I said behind their back. How can I repent for this
That which has happened has happened. The good part is that you have apologized for your doing. This feeling of being still disturbed about it is not bad. It has a lesson for you, in particular, and for us all, in general. Showing mercy even in times of anger is a virtue.
Allah says in the Quran, "Obey Allah and the Messenger, so you may be shown mercy. And hasten towards forgiveness from your Lord and a Paradise as vast as the heavens and the earth, prepared for those mindful ˹of Allah˺. ˹They are˺ those who donate in prosperity and adversity, control their anger and pardon others. And Allah loves the good-doers. ˹They are˺ those who, upon committing an evil deed or wronging themselves, remember Allah and seek forgiveness and do not knowingly persist in sin—and who forgives sins except Allah? [3:132-235]
Here are two articles about this:
In terms of backbiting:
Shaykhul-Islam Ibn Taymiyah said: "Whoever wrongs a person by slandering him, backbiting about him or insulting him, then repents, Allah will accept his repentance, but if the one who was wronged finds out about that, he has the right to settle the score. But if he slandered him or backbit about him and the person did not hear of that, then there are two views according to the scholars, both of which were narrated from Ahmad, the more correct of which is that he should not tell him that he spoke against him in his absence. It was said that he should rather speak well of him in his absence just as he spoke badly of him in his absence, as al-Hasan al-Basri said: the expiation for gheebah is to pray for forgiveness for the person about whom you backbite." Majmu’ al-Fatawa (3/291)
Narrated from Abu Huraira: that he heard the Prophet (ﷺ) saying, "O Allah! If I should ever abuse a believer, please let that be a means of bringing him near to You on the Day of Resurrection. (اللَّهُمَّ فَأَيُّمَا مُؤْمِنٍ سَبَبْتُهُ فَاجْعَلْ ذَلِكَ لَهُ قُرْبَةً إِلَيْكَ يَوْمَ الْقِيَامَةِ)" in Sahih al-Bukhari (6361)
Talking badly about a family with someone else is definitely wrong. However, the good part is that you realized your mistake.
You know... backbiting is one of the worst sins that a person can make. It comes under major sins of Islam.
Allah says in the Qur'an:
“O you who have believed, avoid much [negative] assumption. Indeed, some assumption is sin. And do not spy or backbite each other. Would one of you like to eat the flesh of his brother when dead? You would detest it. And fear Allah; indeed, Allah is Accepting of repentance and Merciful.”
- Surah Al Hujurat Ayah 12
Make Dua sincerely and ask forgiveness from Allah, and focused on your future. Make a mindset that never to repeat such sinful acts, not only related to family but also avoid talking bad about anyone in this world.
Make Allah forgive our sins and save us from whispers of shaitan, Aameen.