What is bad about communism?
Communism is an economic rather than a political system. It was described by Karl Marx as a rather Utopian evolution that would occur after technology enabled the world to provide far more food, shelter, and other necessities than any person would need. Like other Utopias, it can never be tried ion the real world, because (1) over-population means we don't have an economic surplus of the type it requires (2) human nature, especially greed, leads to people wanting to have more than their fair share of goods even if others are starving.
The closest real communities to what Marx envisaged were very small. His models were small agricultural villages, monasteries, and other religious communes where people were willing to hold property in common.
Modern socialist governments like those of China and the Soviet Union are not really "communist"; they combine strongly planned central government with state ownership, and often massive corruption. China's government corporations do seem to be currently the most successive economic units in the world, but they aren't really what Marx was advocating.
There are a few different problems with communism.
First, it assumes that people will work hard simply because they should. In this, it seems to be unrealistic. It assumes that people will work hard and do their jobs well even if they have no financial incentive to do so.
Second, it believes that an economy can be centrally planned. It believes that the government can decide what should be made and how it should be made. It relies on this instead of relying on companies' desire to make money.
Third, it takes away people's freedom. Communism is not a democratic system. It does not believe in allowing the voice of the people to be heard. It does not guarantee rights to its people.
In theses ways, communism is bad both economically and in terms of human rights.