I am not sure that the state has proven, beyond a reasonable doubt, that it was premeditated murder, yet. But they have shown that he was reckless and irresponsible and his actions resulted in her untimely death. So I think he is doing some time, in the end.
OP's own affidavit describes premeditated murder:
1. OP is standing in front of bedroom door that exits the house onto the deck.
2. OP hears a window open in another room.
3. OP gets a gun.
4. OP moves out of room that has 2 doors from which he could exit.
5. OP moves through hallway towards person in bathroom.
6. OP walks into center of bathroom in vulnerable position.
7. OP does not see intruder. OP hears person making sounds in toilet.
8. OP sprays four black talon bullets through toilet door.
That entire sequence of events is, by law, the definition of premeditated murder.
It wasn't an accident. He didn't accidentally discharge the gun four times. He didn't intentionally discharge the gun four times not knowing there was a person in his line of fire.
OP intentionally left a room where he was safe, and from which he could have exited, entered into a second room, and intentionally fired a gun four times at somebody who wasn't threatening him in any way.
Saying, "I thought the person I killed might threaten me..." is not a valid legal defense. If it were, every husband in the world would have legal grounds to shoot his wife whenever she used the bathroom.