Evil exists because God gave us the ability to love. In order to love someone, you have to make the choice to love that person. And if you can make the choice to love, you can also make the choice to not love. Not loving someone is not the same as hating someone (although hating someone is definitely not loving that person).
Because we have this ability to choose to love, there is also the risk of someone making a bad decision. Unfortunately, sometimes these bad choices are evil.
God does not want there to be evil in this world but in order for us to love God and others, there does have to be that opportunity to make a choice.