Maus: what is inference

What is inference?
I infer that if someone is taking 10th grade English that they are in 10th grade.
I infer that if someone is attending a Corning-Painted Post school that the live in the district?
I infer that if someone is wearing an Ohio State hoodie that they are an Ohio State fan?
What inference are implied in the following Doritos commercials?

We infer that the crazed maniac with an ax cuts off the other guy's head.
Why does the author not show the man getting his head chopped off?
    The author leave us to visualize the grisly act in the way it most terrifies each one of us. We became the murderer.