If this crime took place in the USA, what would the sentence be?

More than likely, he would undergo a medical evaluation to determine his mental fitness to stand trial. If declared insane, he would probably be institutionalized for a indeterminate amount of time. If found sane, probably the death penalty or life imprisonment. When Hinckley shot President Reagan he was determined mentally ill, and he's still in a mental institution, albeit they give him several "away" privileges nowadays. This example just for a comparison.