What is the ruling on those who openly insult religion, and what is the Muslim's duty towards them, and is it considered intentional killing if one of them is killed during a quarrel that occurred after he insulted religion?

Reviling the religion is an act of disbelief (kufr) and apostasy from the Muslim community. Students of knowledge should wisely educate people about its seriousness, because the implementation of the hudud punishment is not for individuals, but for the Muslim ruler (wali al-amr) or his deputy. Whoever kills someone for reviling the religion, in defiance of the ruler, shall be disciplined (ta'zeer).