First, you should note that the Old Testament is a sacred text shared among all three Abrahamic religions. This means that your introduction should address the fact that all three religions share a common foundation. Christianity adds the New Testament to this foundation, and Islam adds a third layer, the Koran, on top of the sacred texts of the Christians. Thus, what we see is an evolution in the concepts of evil and what theologians call "theodicy" (divine justice).
The central problem for all three religions is how evil can exist in the presence of an omnipotent, omniscient, benevolent God. The first solution we find in Genesis is a quasi-dualistic one in which Satan is a powerful, evil figure. The second solution, also found in Genesis, is that evil is not an active force but rather an absence of active good. Thus, Original Sin refers to the weakness of human nature. A third solution appears in Job 38.4, in which God says:
Where wast thou when I laid the foundations of the earth? declare, if thou hast understanding.
Who hath laid the measures thereof, if thou knowest? or who hath stretched the line upon it?
This suggests that evil may not actually exist and that our perception of things as evil or unfortunate is due to the limits of our knowledge.
Both Christianity and Islam accept that humans are inherently flawed and that evil arises from those flaws or from a malevolent devil acting upon us. Both religions also believe that suffering is redemptive of evil. In Christianity, Jesus dies for mankind's sin and redeems us from original sin (Romans 5, for example). The Koran emphasizes that evil exists only in this life, and it will not be part of the more important afterlife, in which the good are rewarded.