Final answer:
Ivan the Terrible began acting irrationally after he murdered his oldest son in 1581, which had a significant impact on his mental state and led to a period of increased paranoia and erratic behavior.
Step-by-step explanation:
Ivan the Terrible began acting irrationally after he murdered his oldest son. This event occurred in 1581 when Ivan accidentally struck his son, Ivan Ivanovich, with a scepter during a violent argument, resulting in the son's death.
This tragic incident had a profound impact on Ivan the Terrible's mental state, leading to his descent into paranoia, violence, and erratic behavior.
Prior to this event, while Ivan the Terrible had a reputation for being ruthless and cruel, his reign did not display the level of irrationality seen after the murder of his son.
Learn more about Ivan the Terrible's irrational behavior