There are multiple potential answers to this basic riddle in various religions. Are you interested in understanding them?
For example, we can say that while God is omnipotent, he has chosen to create a world with humans who have the possibility of choosing to be good -- your word -- or not -- and grow through this choice. A world cannot exist where the "evil" is prevented, unless you also take away the the agency from humans and this potential for growth.
Much of which we think of as "evil" is actually consequence of things we do. One simple example in modern terms is that we tend to repeat the same unconscious patterns over and over again.
We often do it because we have a trauma maybe from childhood -- that we are not conscious of. If I get a physical trauma, muscles around the traums tend to form a tension to protect the site where we get hurt. These often become chronic even after the wound has healed. We have many such chronic tensions in our body.
Similarly, when we get a physical trauma that is too unbearable for us to process, we create tensions around it that protect us from experiencing it.
These patterns we repeat cause much of our suffering. By becoming more aware of our wounds, they can start healing, and we can start becoming free of such suffering.
Our "work" is to learn about ourselves, to overcome these traumas, and heal our wounds. In such a case the "evil" we see is actually a reflection which is necessary for us to become aware of these things in ourselves.