Blackwater was hired to provide "protection" for civilian administrators in Iraq, and was the single largest private security firm employed by the U.S. government. The contract they had with the government tops out at 1 Billion dollars. This firm is not subject to the restrictions that keep our national military in check, for the nation as a whole cannot be held accountable for their actions.  Sadly Obama is still dealing with Blackwater. 
Actions for which to be held accountableEdit
Blackwater was founded by former Navy Seals who didn't feel that they had done enough killing in their military careers, and thus wanted to become contract killers. They established a large tactical training grounds in which they train non-military personnel to be able to kill people as efficiently as military personnel can. They then hire themselves out, mostly to the United States government, to kill people that the armed forces don't have enough personnel to bother killing.
While on their supposed protection missions, the Blackwater soldiers were killing civilians indiscriminately. They also smuggled weapons illegally, and they were somehow involved in the debacle that was the Hurricane Katrina rescue effort.
Because they have caused so much harm in the world and broken such a wide array of international laws, rumors developed that George W. Bush planned to keep them under government employ indefinitely.
What are they? Edit
Blackwater and similar soldier of fortune groups are euphemistically called "contractors," which leads people to believe they are building roads or fixing roofs. What they are in fact is mercenaries, people who like war and don't get paid enough/were psychologically unqualified in the military services.