You seem to be conflating killing and murder, in an attempt to equivocate a private actor engaged in non-sanctioned political violence, with a soldier, who has been legally authorized by the proper, democratically-elected authorities to wage war in accordance with our laws.
A great many of our troops are guilty of murder, and should face punishment. The same can be said for our political leadership. But the fact that they often aren't punished doesn't prove that others shouldn't be either - it merely demonstrates the need for us to take these offenses more seriously, and the need to apply our laws uniformly, regardless of social status, wealth or position.
Many of the actions we have taken are murder. Those who perform them are themselves murderers. They need to be evaluated, given a fair trial for those crimes we suspect them of, and finally imprisoned, if guilt can be established.
In regards to these particular fighters, if we cannot prove guilt, and these people are otherwise entitled to be here (ie they have citizenship), then obviously they should be allowed to return. But they will have to face scrutiny first, and if they aren't citizens, be able to prove their own worth.