The defense? Being poor IS a defense and is used over and over and over and over...
Everyone has an excuse and like magic, it always puts the blame on someone that isn't the person who did something wrong. Pretty cool! If we played it out to its logical conclusion, our prisons would be full of the elderly. Instead, the excuse is bought and courts stop short of that, instead slapping wrists of criminals and sending them to go talk out their childhood woes with counselors until we learn our lesson and they do it again. Hopefully that won't be the case with this particular kid...hate that he killed and disabled people, but also...wow, he did NOT set out to do this! Stealing beer to drink vs murder is a vastly different thing. I don't know his state of mind, but I'd imagine it isn't good and that throwing him in prison wouldn't resolve much for anyone.
I can't imagine being on either side of this.
But still...as for the defense? That's not a defense. The lawyer has to try, and blaming the parents is something that has been shown to work in our society whenever a kid does something wrong. Even adults try using that one, with varying degrees of success.