What caused the Khmer Rouge to be so brutal?

There are several illustrations out there about the torture of people in Khmer Rouge prisons, including chopping people's hands off.

This all started as a communist, or pseudo-communist revolution.

What the hell caused them to chop people's hands off? Among all of the other horrifying things they did.

In modern times, I know very few that even rival all the brutality that this regime committed, not by death count but by the sheer inhumanity of the deaths.

I need a detailed explanation of all this.