Also known as Mob Mentality
, it is responsible for quite a few improper actions.
Consider riots and other group/mob actions. The reasons they happen is simply safety in numbers: a person's individual action is hidden among the masses. If a person can't be singled out, then they feel free to do whatever they want.
It also points out the natural direction of human behavior: if someone thinks they can get away with it, they will do it; regardless of what it is.
Humans by nature are selfish and self-serving. The only constraints they have are laws and punishments from outside sources. Left to their own devices they will commit atrocious acts. It's a natural by-product of the survival of the fittest.