In the initial milgram study, the experimenter asked participants to shock someone else whenever that person made a mistake on a learning task. the results of his initial study showed that people

The results of his initial study showed that people were willing to inflict pain on strangers.
According to the study, one major factor that prevent other people from hurting another people is the feeling of empathy and we are less likely to develop a feeling of empathy if we're not familiar with that other people