It's almost certainly accidental. If the bullet went through the back seat, it most likely was an unintended discharge. If this is the case, it's entirely understandable that we would be angry at the shooter, but it's something he'll have to live with for the rest of his life.
My firearms instructor told me years ago that's one reason home defenders should never fire in the dark. It's much better to be injured or killed than to shoot a loved one by mistake.
It's tough to imprison a man for a second of negligence; however, if one has been drinking or is criminally negligent, he'srossed the line.
I had a friend whose two kids found the loaded 1911 that belonged to him. He hadn't carefully hidden or secured it, and after one of the kids accidentally discharged it, the shot went wild. Fortunately it jammed, and the kids couldn't fix it. My friend found the gun stuffed haphazzard into the holster and a bullet hole in the wall, and he got the truth out of his kids. One of the kids, years later, told me that the bullet missed his brother by mere inches.
It was tough on the father and the boy who shot the gun.