There is an underlying assumption in your question that I should highlight since it is a point of delicacy. The assumption is that National governments are concerned with the plight of citizens of another country, they are not. They are concerned with what is best from the point of view of their own citizens. That is not to say that appearances are important, but in this case it is irrelevant.
Support from the governments that you mentioned is strategic from their point of view.
Cuba - Relationship with Venezuela have been close for a long time. Hugo Chavez was a public admirer of Fidel and hope to succeed him as the ideological leader of Latin America and Maduro studied in Cuba. It is often said that Venezuela is not socialist in the vein of Cuba or the defunct Soviet Republic, but they certainly have common roots - Source - globalsecurity.org
Russia - Russia Cuba relationship during the Soviet era is well known and documented. Currently Venezuela is a client of Russia from arms to other exports. They are also military allies conducting joint exercises. Additionally there are some substantial loans. Source - Wikipedia
China - Sino - Venezuelan relationships are not that warm. Other than the socialist training of Maduro, there does not seem to be anything extraordinary that ties them together and I was unable to find any extensive support of the Venezuelan Government. In fact it appears that China is playing both sides (The other side being the US) and courting neither so Someone else may have to chime in.
What ties all the countries support (Except for China) is opposition to the influence of the US. This goes both ways as there are questionable relationships between the US and Asian countries that are not "Clean". I don't want to gloss over that the US is in opposition of Cuba and Russia. This seems obvious with numerous news stories where there is a mini cold-war occurring between these two poles (Numerous warplanes flying too close, Iran, Syria, Africa).
The conclusion is that what is going on in Venezuela is tragic from the point of view of us little people not in government but from the point of view of world leaders they don't really mean much, The same or worse situations are continually occurring throughout the world. A quote attributed to Stalin reads: "1 dead person is a tragedy, 1 million dead people is a statistic". Politicians of major governments have to deal with that every day.