The People's Republic of China's support of Pol Pot make them responsible for his actions. Although the Chinese people did not personally commit the atrocities that Pol Pot is responsible for, their support for his regime implies that his actions and the deaths of 25% of the Cambodian population was condoned by the Chinese.
There are not many people who are responsible for mass murders in our history. In some cases like Alexander and Genghis Khan, they provided great advances in the world and changed the game. Leaders like Pol Pot were monsters in uniforms, and need to be remembered as such, the lack of doing so is inhumane and cruel.
Yes, the People's Republic of China's support for Pol Pot, and his activism in Cambodia, make them responsible for his actions, because they have said publicly that they believe in what he is doing. It would be no different than funding someone to do research, and then trying to distance yourself from the fact that research is being done. We all know China is behind Pol Pot. They are responsible.
I think while it may have boosted Pol Pot in his country to have China supporting him I don't think it really makes China responsible for his actions, it's highly unlikely that China supported him knowing the mass killings he was planning to do to his own people and others.
No, he is still the one that is doing all of the these things, and the people just think that for right now that he is the best leader that they have over there. I think that if he does a bad thing that they will drop him real fast.