In the back???
"In the back" is not a particularly convincing argument either way -- it's just where she hit him; nothing more.
For example, she could be a cold-blooded killer, and shot him by surprise.
Or, he could have been a wife-beater who turned to run away when he saw that she had a gun.
Or, she was tired of him raping her every morning, and shot him when he headed toward the bed.
Or, she's a loon, and shot him when her crazy side took over.
Any of these, and plenty more, could explain why he was shot in the back.
What 'if' she walked in on him doing something and felt it necessary to take action????