They can be ruthless, exploitive, sadistic, destructive, because they have no sensitiveness to the other person's suffering or feeling. This lack of sensitivity comes from lacking the same regarding their own feelings.
Being empathic, means a person is able to sense another person's moods or feelings, as a result of having experienced those same things.
We feel another person's sorrow because it makes us sad too.
We feel another's joy, because it causes good feeling in us too.
But when a person is not able to feel sadness or joy, they cannot respond to such things in another person either, and might even doubt that the other person has those feelings. Denial of our feelings leads us to deny that others feel too.