It's a very good question and a problem that's very difficult to solve. Part of it is evolutionary in terms of survival when the world was a far more more difficult place to survive. Part of it has to do with overpopulation and competition for limited resources. Part of it has to do with social identity. We're social animals, or herd animals if you like. We naturally form groups and identify with that group, are protective of that group, and are naturally disinclined to see people not part of that group negatively. We generally tend to want to feel good and looking down on and insulting others not like ourselves promotes that feeling, and when others do it to us we get angry.
And, as Freud argued, people cope with negative views of themselves by perceiving other people as having particularly high levels of that same negative view. Basically, say you are feeling dishonest. You are then more likely to see other people as dishonest, and this makes you, in a sense, feel more honest yourself. Decades of research supports this. Researchers have discovered that it is threatened self-esteem that drives a lot of aggression.
In other words, it doesn't really matter if people feel good or bad about themselves in general. What matters is that people, in that moment, are feeling worse about themselves than usual. This line of research has found that threatened self-esteem is associated with a wide range of aggressive behaviors. we tend to be more aggressive when our self-worth has been challenged and we are not feeling particularly positive about ourselves. Insecurity over ourselves drives much of the cruelty in the world.
How do we fix that? Mindfulness. More awareness of ourselves and of the sufferings of others (including other creatures. Only when we do that can we practice compassion and gratitude, but it's hard to do if we've been mistreated badly by others for so long (i.e. throughout the history of humanity. Letting go of grudges and the desire for revenge is a tough challenge for many people and groups.
Bit it's not all bad. The world is full of good people. Very good people. If such was not the case society could not survive and progress. When you think of it that way you come to realize that there are far more good people than bad, but it only takes a few t ruin it for everyone. Here's a pretty good set of rules to live by, and most of us learned this before the age of 5: http://www.peace.ca/kindergarten.htm
p.s. Sorry to be critical but it's WE humans, not us humans, as in "why are we just terrible?" instead of "why are us just terrible?".Not too difficult when you understand that "we" is the object and "humans" is just a descriptor.