The real reason he did it was he decided to do it. Any circumstances that in turn affected his decision-making ability (for example if he was himself brutalised in some way) are separate issues. If, for example, let's say his daddy beat him up and made him an unhappy little Sherman, then that is a separate issue and should be treated as such.

Have you never heard the truism, "two wrongs don't make a right"? It seems to me to apply here. And "he's just a kid" is not an excuse here either - that only comes into play when a kid is too young to know that what he did is wrong.