Let me be clear, he did indeed kill someone. All the people did in the series. The shows are about deciding if you think they meant to or not.
In Joes case I am brought to tears. It's pretty much the worst case scenario for a kid growing up in redneck country where they live in tiny shack, have to crap in bottles, get abused, locked in trunks...etc. They treated their child like a mistake because he was obviously mentally slow. No fault of his own.
His whole childhood was constant physical, mental and sexual abuse. No wonder a person who is already having mental issues becomes more mental. Am I saying this excuses murder? No. But it certainly should play a role in his hearing/sentencing. He was bound to do something like this because if his screwed up childhood.
Even if you truly believe he meant to kill that woman, he still needed to be given some slack. This is liking accusing Forest Gump of murder. A person who really doesn't get the world around them. Poor Joe.