I mean, clearly his individual crimes aren't as bad as say those of soldiers raping a 14-year old. But he does willingly and knowingly take part in, and help perform, the the larger crime that is the poorly justified invasion and occupation of Iraq.
There is not such thing as "good" people raping, torturing, humiliating, terrorizing and doing evil things to other helpless and disarmed people.
There's no indication that he directly participated in any of that, and quite a bit to indicate he didn't. After all, he was heading towards pacifism before he was deployed, and became so disillusions that he was honorably discharged as a conscientious objector. I wouldn't be surprised if the people doing those evil things were careful to hide those activities from him. He seems like the kind of person who'd have caused problems for them, and they probably understood that.
In a war of aggression it very much is.
If it is not, then yes it's better to have good people do this job than bad ones, as with any job.