They can quit and leave at any time - nobody is going to extend their "tour of duty", brand them AWOL or court martial them. They presumably knew that this was high risk work and are compensated generously for that very reason.
Other than simple regret that any human being should be murdered for any reason, I resent the administration's suggestion that we should we feel indignation or rage about the murder of mercenaries on the payroll of the CIA, Halliburton, or anyone else there to make big money in Iraq.
I feel great sadness for any soldiers sent into this misadventure who pay with their lives or their limbs, and for their families. But I do not feel one bit of sympathy for "contractors".