Question 1:
There are many examples of this type of behaviour throughout history. For example, during the colonization of the West, Americans would often portray Native Americans as wild, uncivilized, uncontrollable people who were only interested in killing white people. This was commonly seen in tales, comics and newspaper stories. Another more current example is people who demonize immigrants, creating false stereotypes about them or lying about their intentions.
Question 2:
There are several reasons why people do this. Sometimes, it starts as a political strategy (this was to a large extent the case of the Native Americans). As the media repeats the story again and again, it becomes more accepted by the population. Other times, it can be born out of fear of the unknown, as with the case of immigrants. Another common reason is "tribalism," which is a behaviour that encourages strong attachments and support within a social group, but enormous rejection of those outside of the circle.
Question 3:
The effect on society is mostly a very negative one. It can create internal, violent conflicts between different social groups. It can also lead to politicians committing atrocities and not being held accountable for their actions. Scapegoating can also take attention away from real problems and make solutions harder to reach.