Was Mao Zedong a tyrant or visionary?

On the whole, you'd have to say he was both. Mao was indeed a visionary, a man with a radical vision of what he wanted China to be. For centuries, China had been routinely exploited and taken advantage of by a succession of imperial powers, most recently Japan. Mao wanted to put a stop to all this; he wanted China to take its place among the international community as a strong, independent, communist state which would no longer be pushed around.
Unfortunately, the implementation of Mao's radical vision involved human suffering on a huge scale. The People's Republic of China, established by Mao, was and is a one-party state. Right from the start, anyone deemed to be an enemy of the regime was in danger of being tortured, imprisoned, or executed. Mao encouraged class warfare in the Chinese countryside, with poorer peasants being pitted against their landlords. As well as leading to mass killings, this campaign resulted in the disruption of the harvest, causing the first of many avoidable famines under Mao's regime.
Although Mao briefly encouraged people to speak out and criticize the Communist regime during the so-called Hundred Flowers campaign, his tyrannical instincts never really left him, and repression soon returned with greater force than ever. Mao's tyranny reached its nadir during the Cultural Revolution when Mao and his acolytes incited open rebellion against the leadership of the Chinese Communist Party in order to regain some of the power and influence he'd lost as a result of the disastrous Great Leap Forward, a failed effort at mass industrialization. During the Cultural Revolution, China was plunged into total chaos and anarchy. Tens of millions died. But amid all the turmoil, Mao's tyranny grew stronger and stronger as he unleashed the revolutionary energies of China's youth, the very same revolutionary energy with which he established his Communist dictatorship.