This is an age-old question and different people can answer it in different ways.

One of the most typical answers is that God has allowed all people to have free will.  God had to do so in order to allow people to be good.  If we do not have the choice to be evil, then our good actions have no moral value.  But if we have the choice, then it can be to our credit when we are good.

For this reason, God gave us the ability to be good or evil.  Sadly, some of us take advantage of the opportunity to truly be evil.