This is a question that many non-believers pose to themselves internally. It is usually used as an easy reason to dismiss the idea of following a religion when there is no other reason that can be thought of to do so. Since God allows people to commit acts that we deem to be evil, the conclusion drawn is that God cannot be good and is therefore not worthy of our attention. This is a logical fallacy found merely in the definition of the word good.
This is a tough question that many Christians and Atheists alike can find themselves struggling with. Challenging questions like this are not something that I think we should ignore or run away from as they tend to embed themselves into the back of our mind and pop up at the most inconvenient times. This is why I think we should embrace issues like this and work through them together as a community of believers.