I have the two best biographies of Pol Pot in English: Brother Number One by David Chandler and Pol Pot - History of a Nightmare by Philip Short.

We all agree that he's Beta. I certainly can see the case for ISTj and I would suspect that it's based on Pol Pot "staying out of the limelight" and being secretive for a very long time, like Stalin. I think that can be explained largely by the Khmer culture with its Buddhist characteristics where self-effacement is a virtue and the way to gain respect (points made in both books).

I think he was more likely ENFj because the one thing everyone who ever met him agrees on - at all stages of his life - is that he was extremely charming and charismatic in personal contact. Even Prince (on-off King of Cambodia) Sihanouk, who I'm convinced is INFp, said that about Pol Pot, which is not how an INFp would likely describe an ISTj.

Also, he was not a Stalin-like, ISTj bureaucratic dictator - - Pol Pot was concerned with his "vision", not with details. When his goals were not achieved, be blamed those he had delegated to carry out the details.

Up to the end, his chief weapon was emotional manipulation - - in the last major interview he gave, before his death (unfortunately it's not fully available), his arguments were of the sort, "my conscience is clear - - if you look at me, can you say I am a savage person?" etc - -

In a nutshell, the impression that everyone had of his personality is of a engaging, charming person in close contact -- that was his chief characteristic according to everyone. His "rule" of Cambodia was more based on pursuing his grand visions than getting involved in annoying details.

I could quote lots of eyewitness descriptions of his personality from the books, but that would be too much work and take too much time.