An animal who suffer from intense stress because its odds of survival are limited compared to the competition in any given environment might show some aggression toward other animals in its species. Failure to dominate other males and females might frustrate it(him).
Some men call this a "mental illness" to label other weaker men who are lured into believing they are abnormal. The very same (educated, loved, raised to dominate other weaker men in society) men who label others as mentally ill laugh all the way to the bank selling them pills.