I think that, in the beginning, he may have believed that it (Pol Pot’s genocide in Cambodia) was all a frame-up by the New York Times and the US-Nazi alliance or whoever else he made up to blame it on. No doubt a great deal of wishful thinking on his part was involved, but it’s possible he was sincere in his conspiracy theories.

Then, as the facts became more difficult to deny and he started looking worse as a result, things got more complicated. At some point, he must have realized that he was saying things that in all likelihood were false. My guess is that he justified it in two ways: First, by relativizing it. Something along the lines of “whatever the Khmer Rouge may have done, it can’t be as bad as what America did in Vietnam, or Chile, or Indonesia, etc. Therefore, I am justified in continuing to defend the regime.” Second, by demonizing his opponents, by saying “whatever the Khmer Rouge may have done, it’s more important not to allow my opponents to win, because they are evil, and it is morally wrong to allow evil to win.”