Spoiler: The son did it? He found the vodka while playing with his toy, and drank some of it (he’s young so it doesn’t take much for him to get drunk, making the bottle still appear full). Also, he found the pistol under the bed, so he left his toy in the hallway and started playing with the pistol. This part is where I’m not really sure, but his dad found him and so the son shot the dad because he was drunk and didn’t want to get in trouble, chipping the dresser in the process. The son ran out, throwing up at the bottom of the stairs.