Aggression:

 "An aggressive action is intentional behavior aimed at causing either physical or psychological pain."

"Hostile aggression is an act of aggression stemming from feelings of anger and aimed at inflicting pain."

"Instrumental aggression is aggression that serves as a means to some goal other than causing pain."

"Both animal and human studies show that pain will increase the probability that an organism will aggress. Other forms of bodily discomfort (heat, humidity, air pollution, offensive odors) may also act to lower the threshold for aggressive behaviors."

"Frustration-aggression theory says that frustration, the perception that you are being prevented from obtaining a goal, will increase the probability of an aggressive response."

"People usually feel the need to reciprocate after they are provoked by aggressive behavior from another person."