Why is capitalism considered a bad thing by communist?

Communists are somewhat Lockean in their idea of production at least Marx was. They believe that mixing your labor with something makes it yours in a sense. However capitalists are those that make money off of others' labor. They merely own the means of production and take their living from the surplus value of the labor they hire. This is a very crude answer to your question, but I hope it points you in the right direction as to why communists have moral issues with capitalism.