What his sin is seems to have more to do with his attempts to get out from under Anglo-American neo-colonial serfdom dependency and to pursue a national economic development independent of the International Monetary Fund and World Bank.
I think any reasonable person would probably agree that Mugabe's biggest sins include, in no particular order, (1) taking a nation with a thriving agricultural base that was an exporter of food and turning it into a nation on the brink of famine, (2) using violence and intimidation to brutally supress any political opposition, (3) dismantling any semblance of democracy, (4) presiding over the utter collapse of the economy, and (5) causing untold suffering for the people of Zimbabwe, who had every expectation that by this point Zimbabwe could be a leader and symbol of progress in Africa.
The fact that some douche-bag leaders in other countries might also dislike Mugabe because he's doing business with China is utterly irrelvant. He is still a world-class asshole that has done his people a horrible disservice by remaining in power.