Why are people so cruel

Usually because they feel powerless, jealous, confused, or they've yet to understand things a certain way. Some people kick inwardly and feel self-critical and some people kick out at people around them to make themselves feel better (which doesn't work because it's not exactly the positive self-image they're aiming for).

The answer is not simple.

Lack of empathy and a strong sense of arrogant entitlement (superiority) which is increasingly evident in societies which are obsessively focused on the rights of the individual as opposed to the bonds of the community. Social dominance, power, and self-esteem maintenance are other factors. Biological factors, like low levels of serotonin and higher levels of testosterone correlates with aggressive behaviour too.

As for people picking on differences, it comes down to an us versus them mentality. If they feel a threat to their own identity or have a particularly strong identification with their group (whatever it might be), it can increase their sense of belonging and their self-esteem to discriminate against anyone who is outside of the boundaries of the group they identify with. People who feel defensive or threatened are more likely to engage in cruel behaviour to boost their self-esteem. Although, most people know it's wrong to do it and have more self-control when it comes to taking it out on others.

It can only be reduced by promoting a common identity and an alternative route to boosting self-esteem.

It's mostly about sexual frustration.

Dappled said it very well, and I would add that it's in our culture now, for a variety of reasons. We don't value diversity, but I do see things changing, however slowly, as evidenced by you even asking the question. Good for you!

As a lesbian, I've really felt the cruelty of the world. People can just be mean....and thats just part of life i guess.

People are scum that find it amusing to torment those who may be different. They are disgusting, inhumane jerk wads. I imagine their abusive natures stem from their own burning desire to make others feel bad to make themselves feel better about their own appearances and/or egos.

Ppl are by nature twisted selfish beings. But for every jerk, there's a saint. It could be much worse. But your right it is a problem.

Why people believe them is mostly due to complacency and laziness.
They do not challenge them.
They do not do any type of research and establish the facts of the matter. They quite often accept the status quo as being the ultimate authority and sometimes superstition and ill founded beliefs prevail.
The reasons for 'authority' are varied from society to society and often from jurisdiction to jurisdiction.

http://en.m.wikipedia.org/wiki/Authority

There are also many who pretend to be an authority on certain matters, but in reality, actually know very little about the issue at hand.

I think people are cruel to each other because some people just plain out deserve it. It's that simple.

"People" are cruel because "people" are ugly.
And OP is people.