whoops, I read your question wrong. No. Sorry. God waited and got the last 4 out of Soddom. When he sent the rain again in Israel, he sent the prophet first to the rebellious king to call him yet several more times to repetance, pursuing him with love until the final moment. God does not "let" anyone go to hell, but they do go. This makes the witness more urgent, not less.
Your answer is a little vague to me, but I think you said that none of the 2 million Khmer victims would have become Christians had they not been murdered. Am I correct?
Yes. I must assume that any who did not seek Him in time would not have.(Exactly what happened in those last moments is another question.)