I don't know if you're joking, but Atticus literally says 'I was just following orders', and it seems to be an excuse that most fans accept because he's the real victim (he's experienced the trauma of racism, you see) and not the civilian women he guns down as they beg for their lives, and certainly not their families or loved ones. I'd point out that this (the war crime) is also an invention of the TV show, in the book he's simply a returning Korea vet.
Incidentally, he's kind of shitty at war crimes too. He's looking for information, but he shoots two of the women before he even starts asking for it, so there was a good chance that he killed the person who could have given him in the information he needs. It's only luck (he shoots the wrong two women first) that stops him from executing them all, including Ji-ah.