Why are some people mean?

Because they want to be.  I was shaping up to be a fine little bully when the mother of one of my victims laid down the law to me.  I was shocked -- bullies were the people who tortured me.  We tend to repeat what we experience.

And some people simply lack love for God and his human children.

It is part of the prophecy mentioned in 2 Timothy 3:1-5, that mentions, “But know this, that in the last days critical times hard to deal with will be here.  For men will be lovers of themselves, lovers of money, boastful, haughty, blasphemers, disobedient to parents, unthankful, disloyal,  having no natural affection, not open to any agreement, slanderers, without self-control, fierce, without love of goodness,  betrayers, headstrong, puffed up with pride, lovers of pleasures rather than lovers of God,  having an appearance of godliness but proving false to its power; and from these turn away.”

They're mad because they've been hurt, so they take it out on others.

They are children of the devil

1 John 3:10 — So now we can tell who are children of God and who are children of the devil. Anyone who does not live righteously and does not love others does not belong to God.

Poor upbringing usually.

Usually for a good laugh or to mask their own insecurities. They might also be mentally ill or on drugs.