Actually, right at the time of his death, there were strong indications from the Khmer Rouge that they were considering turning him over to the international community for a tribunal. Unfortunately he died before this could happen, so from that regard it's really a timing thing.

Now, if you're asking as to why he was never brought to justice earlier, I'm a little unsure of how that might be done. When these atrocities were happening, it was when Pol Pot was in control of an entire nation. Even after he lost power, he didn't retire to some townhouse but instead fled with the Khmer Rouge to establish a military resistance. So I mean in those regards, it's really a logistics type thing. In terms of modern comparisons, Kim Jong Il was never "brought to justice" for his alleged crimes against humanity, and even Kony, who was indicted by the ICC, is still running around in the jungles. It's not as if they could send police officers to his door; it would involve an entire military campaign (which was in fact being conducted against him when he was exile, all the way to his death).

So I mean to me, it seems much easier said than done to suggest that we "bring him to justice." I suppose there is also the implication of why the international community didn't send a task force or more aggressively oppose his regime or hunt him after his fall, but there are obviously a ton of geopolitical and diplomatic issues at play there which I don't think I should speculate on. I hope this helps answer at least part of your question; to be honest, I wasn't entirely sure what you were trying to get at.

As a followup, why was he allowed to live in a villa in Thailand guarded by the Thai military?

The entirety of the explanation below is taken from the book A Problem From Hell: America and the Age of Genocide by Samantha Power.

So justice can be viewed through several lenses, I will break apart the scenarios examined in the book:

Why didn't the US intervene militarily as supported by some american politicians (most notably George McGovern and others).

First, many people in the US were opposed to any military actions in Southeast Asia because of lingering feeling of distaste left by the War in Vietnam, keep in mind the KR started coming to power as early as 1974.

Second, the KR's main killing period was only ended in Dec of 1977 when Vietnam invaded the country with 60,000 troops, who were later also aided by Soviet reinforcements, this caused many Americans to actually back the political legitimacy of the KR in the hopes to stem the spread of communism.

China would later back rebel forces against Vietnam and the USSR.

This strange support of the KR was so strong that they formally held Cambodia's seat at the UN until 1982, and even after that were an active part of the coalition holding the seat that was made up of KR and other non-communist and Chinese supported resistance factions until 1989, when the KR flag was finally taken down from outside the UN.

Basically, even though the KR was stopped from the all out insanity of 74-77, their fighting and killing continued for a fairly long time.

Next we examine why the UN didn't do more.

Strangely enough in 1985 a UN investigation concluded that the KR had committed acts of genocide "...even under the most restricted definition."

I will repeat, the KR flag flew outside of the building until 1989 so take that for what it is worth. If you are interested in more about the obstacles the international community faces you are going to have to read the entire book.

The short version is that actors like the International Criminal Court, The International Court of Justice and international laws covering genocide were not robust functioning entities during the time period we are discussing.

Much stronger attempts were made in the mid to late 90s to bring KR actors to justice, but I don't want to break the 25 year rule.