I would imagine it was because of a combination of events..perhaps as because he learned he had to be cruel from an early age? And he was an alcoholic...but maybe the mercury didn't help

" After years of persecution by the Shuisky family, the poisoning of his mother Helena, and the banishment of his only friend Vorontsov, the thirteen year-old Ivan invited his rival Prince Andrei Shuisky into his room. No one feared the young boy and the boyars often ridiculed him. When the guards were suddenly ordered to seize Prince Andrei and execute him, no one saw it coming."

Source: BOOK 48 laws of power