Why did Ivan the Terrible become a MONK before his death?

 A few years later, he sent a letter to the monastery, in which he recalled how the description of monastic life "rejoiced my vile heart and wretched soul, for I found a bridle for my intemperance and a saving refuge in God's help".

He fulfilled his wish before his death, becoming monk Jonah, which enabled him to be buried in monastic robes. According to another version, Theodosius Vyatka, Ivan the Terrible's confessor, performed this rite over the monarch when he was unconscious after an apoplectic stroke.

The tradition of taking holy orders before death came to Old Russia, along with other Orthodox traditions, from Byzantium. Prince Alexander Nevsky was one of the first to take holy orders, becoming monk Alexy. In 1547, he was canonized and, in 1724, by order of Peter I, the relics of the holy and faithful prince were transferred to the Alexander Nevsky Monastery in St. Petersburg.

It’s believed that this act erased all a person's, “dying” them to worldly life and reviving them in the spiritual. This means they will not be judged for sins committed before taking holy orders. Therefore, for Ivan the Terrible, a deeply religious man on the one hand and an extremely cruel ruler on the other, taking holy orders was important. It was his way of demonstrating his humility and repentance before God.