Aggression can be either physical or verbal, and behavior is classified as aggression even if it does not actually succeed in causing harm or pain.
Behavior that accidentally causes harm or pain is not aggression.

Property damage and other destructive behavior may also fall under the definition of aggression.

Aggression is not the same thing as assertiveness. Aggression is a perplexing phenomenon.

Why are people motivated to hurt each other? How does violence help organisms to survive and reproduce? After two centuries of theories and technological advances, psychologists and other scientists have been able to look deeply into aggression's biological and evolutionary roots, as well as its consequences in society. The area from which all emotion originates is the brain.

While scientists continue to test various areas of the brain for their effects on aggression, two areas that directly regulate or affect aggression have been found.

The amygdale has been shown to be an area that causes aggression.
Stimulation of the amygdale results in augmented aggressive behavior, while lesions of this area greatly reduce one's competitive drive and aggression.

Another area, the hypothalamus, is believed to serve a regulatory role in aggression.

The hypothalamus has been shown to cause aggressive behavior when electrically stimulated but more importantly has receptors that help determine aggression levels based on their interactions with the neurotransmitters serotonin and vasopressin.

Science Daily