The most despicable word known to mankind: 'power' .
Because they believe they can get away with it.
They feel stronger because of the numbers. Also, when some feelings are shared they increase. Stupidity is a feeling lolz
For some bizarre reason, when ever people join together in a group, they will always want to put the blame on their woes on the odd person.
I would have to say that it is due to the power and safety in numbers, and the vulnerability of the individual.
They don't care if it's "moral". They are serving a larger agenda and will often do whatever it takes to shift the blame or accomplish what they want.
Because they are all secretly envious and gutless so they find power in numbers and are all united in their pure hatred and jealousy for that one person when in actual fact that one person is better and stronger than they'll ever be!!!