Communists don't have a great reputation for peace.
China is an authoritarian dictatorship with their own flavour of communism I guess. Blaming everything they do on commmunism as a whole is very uninformed.
Add that to the tankies I've met personally and their beliefs, and yeah, communism sucks.
Don't get me wrong, I can't really stand uber-authoritarian Communism. But I don't see how communist/marxist theory is bad as a whole. Would certainly help erase a lot of issues we have on our earth. Unfortunately, people at the top always start to misuse their power to stay there. But that's also the case in capitalist America, so then again, why is communism such a problem for many americans?