It seems that monkeys are just like humans, emotional. In any case, they do not like the insults, especially the fingers of honor.
See what happens when you disrespect a primate:
Never tame animals, when they are not doing you any harm.
A Young man in Shimla learned it through a knockout blow by a monkey, who didn’t take well the middle finger he was being shown. This hilarious as well as surprising footage of a monkey reacting after being instigated by a boy was captured by a CCTV camera near D.C. Office Shimla. As you can see, the boy looks
straight into the eyes of this monkey who is sitting near a bench, where other people are also seated and provokes the animal by showing middle finger. This is seen as an open challenge by the monkey, who knocks the boy down with a wrestling like move.
No injuries were sustained to the boy after this incident, but the humiliation must have hurt him duly. Perhaps, he learned it the hard way that animals should not be disturbed, when they themselves are not unsettling
or attacking you in any way!