Some people are bitter because something has happened to them. They probably believe that it is someone else's fault for their misery. Or maybe they are just so unhappy with themselves that they do not believe that others deserve their own happiness.
Often have been exposed to dealing With life in a negative way growing up Some people are sociopathic, "without feeling for others"usually in the Genes And worse " psychopathic"also a genetic disorder, these people have NO feelings for others, have NO empathy, but can be clever enough to Fit in, then they can turn that switch And they can be mean, often delight In physically hurting animals, other People and with no regard for them. Bottom line they have no consideration at all for anyone unless It fits their needs