One of the main reasons is that these kinds of people like to feel superior and make themselves look better. Another reason is that they want attention and want to gain control among their peers. The best thing to do is to just ignore them to take away that attention from them.
Because they are brought up that way. They are really shameless creatures. They play with emotions and sentiments of underprivileged, they are real jerks, they feel happy, when they see people suffering. They are worst people on earth.