Why did Mao Zedong kill people?

When a revolution occurs, a new group takes power and they sweep away all the previous power structures and rulers of a country. Not surprisingly, these people want their power and money back and the new regime has to defend itself, usually leading to great bloodshed.

History shows that the new leaders usually go overboard and see enemies everywhere, and they attack anyone who could even possibly be a threat, even other revolutionaries who might be popular alternatives.

Mao's forces killed the usual enemies of communism: landowners, rent seekers, some businessmen, employees and members of the old government. Ten years after coming to power, a failed experiment in agrarian reform saw millions die of starvation. A few years after this, Mao unleashed Red Guards, mostly young people, to kill, denounce, imprison and disempower most of the officials who had flourished under communism. Thus a while generation of potential rivals to Mao were killed off by mobs of young people, called 'Revolutionary Guards'.

Along with land reform, there were also campaigns of mass repression and public executions targeting alleged counter-revolutionaries, such as former KMT officials, businessmen, former employees of Western companies, intellectuals whose loyalty was suspect, and significant numbers of rural gentry.