Struggling with Hell
“I can’t imagine someone burning in hell simply for not accepting Islam as a religion…”
No one asked you to.
With his very own eyes, the Prophet (peace be upon him) actually saw people burning in hell during Israa’ and Mi’raaj. However, because he spoke to Allah and knew His glory at a level that we do not, he never asked Allah to explain why hell was so “gruesome” and “barbaric.”
Our minds require a serious reorientation. We fail to grasp who Allah is and the magnitude of the crime of kufr. Instead of complaining about why our minds cannot grasp these things, let us try harder to get as close as possible to a prophetic cognizance of the Glory and Majesty of Allah so that we can truly appreciate the magnitude of the crime committed against Him (i.e., kufr).
Further Reading: The Rationale of Hell in Islam (lecture transcript)
How do you reconcile
لَيْسَ لَكَ مِنَ ٱلْأَمْرِ شَىْءٌ أَوْ يَتُوبَ عَلَيْهِمْ أَوْ يُعَذِّبَهُمْ فَإِنَّهُمْ ظَـٰلِمُونَ ١٢٨
وَلِلَّهِ مَا فِى ٱلسَّمَـٰوَٰتِ وَمَا فِى ٱلْأَرْضِ ۚ يَغْفِرُ لِمَن يَشَآءُ وَيُعَذِّبُ مَن يَشَآءُ ۚ وَٱللَّهُ غَفُورٌۭ رَّحِيمٌۭ ١٢٩
and other such ayah of Allah's Mercy appearing several other places in Qura'an?
I am very hopeful about وَٱللَّهُ غَفُورٌۭ رَّحِيمٌۭ ١٢٩
And very, very hopeful about
۞ قُلْ يَـٰعِبَادِىَ ٱلَّذِينَ أَسْرَفُوا۟ عَلَىٰٓ أَنفُسِهِمْ لَا تَقْنَطُوا۟ مِن رَّحْمَةِ ٱللَّهِ ۚ إِنَّ ٱللَّهَ يَغْفِرُ ٱلذُّنُوبَ جَمِيعًا ۚ إِنَّهُۥ هُوَ ٱلْغَفُورُ ٱلرَّحِيمُ ٥٣