As a former Army officer and Vietnam veteran, I was appalled by these unwarranted pardons. War is terrible enough without making acceptable behaviors that challenge good military order and discipline.
Using armed civilian security operatives in military combat roles is always a bad idea. Not only does it cost America far more than employing the regular military (these men were paid 2 - 3 times more than the equivalent US military personnel, not to mention the extraordinary profit margins of companies like Blackwater) but they do not operate under the same scrutiny and discipline as real American soldiers, sailors, and airmen.
Combat is a dirty business, best handled by people with a strong moral compass and strong rules, traditions, and training. The US military is diligent in training and supporting such people. History is full of atrocities committed by mercenaries such as Blackwater.
America has been diminished by the actions of these men and pardoning them was an act of ignorance and moral bankruptcy.