The problem of evil is one of the classic problems of Philosophy of Religion. How can a God who is omniscient, omnipotent and completely benevolent allow evil to exist? The most obvious response is that such a God would not, and since evil does exist, God, as characterized above, must not exist. The problem of evil is for those who wish to avoid this conclusion. How can one explain how both a benevolent, omniscient and omnipotent God can exist as well as evil? St. Augustine was the first to propose characteristically Christian response to the problem but Christian thinkers have been considering the problem ever since.
The 17th and 18th centuries in Europe represent a period in which religion was much more on the mind of philosophers and was much more prominent in the public sphere of life than it is in Europe and the United States today. The problem of evil was thus of absorbing interest to many philosophers during this period. Here are some suggestions about how to approach this problem.
Does Voltaire accurately represent Leibniz's position as to the solution of the problem of evil when he mocks it in Candide or is he just making fun of a straw man? The real issue here is about how well Leibniz does in solving the problem of evil. Your real problem, then, will be to understand Leibniz' Theodicy. C.D. Broad's Leibniz may be useful among other works on this topic.
One might also explore the problem of evil by considering Hume's Dialogues Concerning Natural Religion. You might want to consider the vexed issue of how much Hume concedes in the end to the argument from design.
Hume's Chapter on miracles (Chapter X) in the Enquiry Concering Human Understanding is another topic for a paper. It is also one which commentators argue over. Your research effort will be to find commentators with conflicting views. Then your problem will be to determine whose interpretation is right? How effective is Hume's argument?