I guess I see why some ppl didn't like it. The movie has many mature elements that makes the audience see it as aimed to mature audience. Those elements don't fit on a game created by a kid, so it feels odd to then learn it's a kid's game.

In fact that's the thing I didn't like about the movie, that a kid would create elements like Baker's and Constance's complex relationship, and slight reference to sugar daddy.

Even so, I interpreted the plot as the kid being very smart and havind dedicated so much time and effort in creating Baker that he ended up becoming and sentient AI.

He's sentient but he's not aware he's in a game. So as he becomes smarter he end up figuring that everybody else are NPCs with lower intelligence. I guess Miller was a special character created by the kid to send a special message to Baker, and knowing it was a game, which ended up hinting that to him.

I guess that, as the abuses advanced, the kid got in the dillema of killing or not the guy. That was in fact the kid's dillema and not Baker's. The kid knew it was wrong but he believed it had to be done, so changed the game from Baker hunting the big fish into bringing the dillema in there, and having Baker (a very advanced AI) reflect about it and decide what to do, then the choice Baker would make would decide what the kid would do. For some reason the kid had some of the traditional NPCs and even added Miller to persuade Baker to not do it.

When Baker ended up killing Frank, the kid had his answer and also killed him.

The ending is very confusing. It seems that the kid wasn't arrested because it was considered self-defense. So, after he was released, he decided to finally talk directly to Baker than add himself to the game, so he'd play with his father.