I see far greater similarities between IS and the Nazi's than with the Khmer Rouge, tbh.
Other than IS sharing a propensity towards butchery with the Khmer, the likeness is hard to find.
The Khmer Rouge were an insular isolated grouping focussed solely upon domination of their own patch. On the other hand, IS -- like the Nazi's -- is driven towards expansionism. For the Nazi's it was a aspiration to dominate Europe at the very least; equivalently, IS is motivated towards establishing an intranational Caliphate. Both ideologies are/were structured on a hunger for world supremacy on their own terms. Both of them tyrannies and both despotisms with a global slant. Conversely, the Khmer Rouge was nationalistic and xenophobic in its outlook.
Then there is the issue of cult-worship in the style of a mediaeval belief-system -- a dated interpretation of Islam on one hand and the Teutonic paganism of the Nazi's.
Both Nazi's and IS also compel the ritualistic idolization of a single figure -- Hitler in one case and al-Baghdadi in the other. Any hint of dissent to the autocrat or the theocrat is ruthlessly eliminated. Oppositely, the Khmer Rouge was organisational and institutional in its architecture.
The most glaring similarlity between Nazism and its Islamofascist brother IS is an absolute hatred for Jews.
Of course, there is also the matter that whereas Nazism was extremist Right Wing, the Khmer's were extremist Left Wing in philosophy.