Communist Beijing is dead set on maintaining its dictatorship over China, and Hong Kong. It will not allow anyone who is not under its control to take power in China, because its very existence depends on it. By extension, Beijing will not allow it to happen in Hong Kong. Hence it is not going to accept any election arrangement under which it is possible for someone not under its control to be elected. Hence it is not going to allow any such person to be nominated.
Hence what the “Occupy Central” gang is asking for is akin to asking a tiger for its skin - 與虎謀皮. Without its skin, the tiger will die. By the same token, without its power, the Communist party is as good as dead. It is not able to compete fairly in a truly democratic, open society. So it won’t. That’s why it is so intransigent.
We should not give up striving for open elections and true democracy. But violence is not a viable solution. It is a hard place to be in.