Yes he chose to do it. Because this choice comes from his mental state, which was obviously not in a stable state. The type of murder shows this. It's an act of rage. Rage which has been built up in his mind because of his experiences growing up. I'm not saying his mum is solely responsible. Like I said, it would be worthwhile to look at the environmental factors. These make a big part of who you are as a person as you grow up. He's 16. No 16 year old, who's grown up with love and care would bludgeon the person who brought him up like that. Something is clearly wrong in the his upbringing.