Although quite far down the list of pokemon which pose a danger to humans, Mankey as a species are unmatched in their aggressiveness and ferocity. They are both brave and brutish; they will pick fights whether their battles would be easy victories against weak and often unwilling foes or impossible struggles against legends. Their courage has won them many accolades, which they typically squander just as quickly by fighting just as ferociously against young children who wander into tall grass.
Because of this ferocity, they are typically considered to be dumb beasts; in reality, they are nothing of the sort. As far as non-psychic pokemon go, pokemon neurology has demonstrated convincingly that Mankey possess brains nearly as developed and intelligent as humans. It is commonly thought that the combination of a self-aware brain and a body which can only run, punch, and repeat its name creates a great deal of frustration which drives Mankey to rage against everything in sight.
There are, however, those who believe that Mankey's aggression does not stem solely from frustration and existential angst. After all, Mankey's closest living relative (apart from Primeape) is Man, which is no stranger to assault, murder, and war.