If you want to know why someone would want to be a Communist, I guess the best thing to do would be to read books about different people throughout history who decided to be. People like Che Guevara, for instance. There are plenty of good reasons why people do the things they do. It depends a lot on the circumstances around them. Most of the people who became communists did so because they genuinely thought it would help the oppressed people in their society and bring down the rich oppressors. And sometimes...it did. Depends when and where.

Communism, like anything else, can be used for good or evil. Same goes for capitalism.