Anybody grappling with the Problem of Evil will have great fun with John Milton's Paradise Lost. Milton's self-stated goal is to "justify the ways of God to men", which includes an explanation of the necessity of evil, perdition and all the rest, in the epic style of Hellenic poets.
In the mean time, I'll gladly come to the defense of theism and provide a specific response by suggesting that the omnipotence of the divine (which in any case, is not explicitly attested in scripture), need only include the ability to perform every act which is possible - I venture that this is an exact explication of the term "omnipotent": i.e., possessing every possible power.
Next, I apprehend that the prevention of all evil is impossible, it is not part of the set of all possible powers. The prevention of all evil is an impossibility because, according to Christian doctrine, the imperfection of humanity is intrinsic to its nature since the fall - "To err is human, to forgive, divine".
The deity retains omnipotence if it cannot perform an impossible act, because impossible acts are not within the scope of performance, by definition. An omnipotent, omnibenevolent demiurge can quite easily (must, in fact) bear passive witness to the unspeakable atrocities of Earth and Hell, because both are the consequences of the volition of humanity (or in the latter case, of angels), and for things to be any other way is inconceivable - humanity without its moral imperfections would not be humanity (and Lucifer without his vanity would not be Lucifer). Jehovah is under no obligation by omnipotence to do the undoable - and altering the essential nature of humanity without effacing its identity is not within the set of performable acts.