I don't think the people who did this were even Iraqi. The news says it was a Jordanian. "This is revenge for the prison abuse" was likely just a convenient excuse. Didn't they also capture and behead a reporter, a while back? Why did they do that?

If my country was occupied, I would not capture civilians and murder them and put videos of it on the internet, no. Any reason other than that those people are sick, evil freaks, is a justification and an excuse.