Hetty tells Callen that in 1944 his grandfather went to Romania to "help King Michale I overthrow the fascists who'd sided with Hitler". However this historically impossible.
King Michale I didn't overthrown anybody, he was the king.
He simply explained the situation to his parliament and military leaders, had his Prime Minister (a friend and loyal supporter of Hitler) imprisoned and took over control of the army. It took less than a day.
Up until 1944 Romania was part of the Axis and fully supported the Nazis. Until King Michale I realised that the Axis couldn't win the war and switched sides to protect his country.
Hetty tells Callen that his grandfather, working as a CIA operative, executed Comescu for war crimes. However, even war criminals are entitled to a trial.
What the CIA ordered was Extrajudicial Execution (ie execution without trial) which is on par with war crimes according to International Law. Both are crimes against humanity.