What is the ruling of Islam regarding someone who quarrels with a person who has insulted him and his parents, and the quarrel escalates to killing? And are both of them considered to be in Hellfire? And is self-defense obligatory, even if it leads to killing? And what is the stance of Islam on swallowing insult and remaining silent about it?

The fundamental principle in Sharia is that punishment should be proportionate to the crime, as Allah Almighty says: (And if you punish [an enemy], then punish with an equivalent of that with which you were harmed). So, whoever exceeds the permissible limit deserves punishment. Although Allah has permitted the believer to retaliate in kind, He has encouraged patience and forgiveness due to the great reward therein. Allah Almighty says: (and those who restrain anger and who pardon people). And whoever restrains anger while being able to unleash it, Allah will call him on the Day of Judgment to choose from the Houris (maidens of paradise).