Subhuman words and actions aren't worthy of a human being. Stealing a toy from a child is a subhuman thing to do.
Sub- means “below” or “under.” Just as a submarine is below the surface of the ocean, something subhuman is below what is appropriate for human beings. In science fiction, creatures from another planet might be described as subhuman if their civilization is less advanced than ours. Here on Earth, subhuman is often used to describe cruel behavior. Unfortunately, humans do lots of subhuman things.