Is a person held accountable on the Day of Judgment for words in which they expressed their opinion, such as a group deserving to be killed, and for thoughts, reflections, and ideas that crossed their mind but they neither uttered nor acted upon?

A person does not perform an action or utter a word except that angels are observing it. Allah, the Almighty, said: "But indeed, [appointed] over you are guardians, Noble Recorders, Who know whatever you do." And He said: "He does not utter any word except that with him is an observer prepared [to record it]."

The Prophet, peace and blessings be upon him, said: "Whoever believes in Allah and the Last Day, let him speak good or remain silent." And he said: "And are people thrown into the Hellfire on their faces—or he said on their noses—except for the harvest of their tongues?" And he said: "Indeed, a servant may utter a word that displeases Allah, not attaching any importance to it, by which he falls into Hell."

This indicates that a servant will be held accountable for the sins he says and does, and will be judged for them. This includes his judgment of people as deserving of death if they do not lawfully deserve it.