I think the issue of "why is there evil," is confused with existence of God. Also I think in the West, a metaphysical structure of the world was denied beginning in the Middle Ages with the idea that universals do not exist and getting up to the British empiricists. Both of these issues I think tend to obscure the central issue.\
The people most likely to use the "problem of evil" as an argument against Christianity tend to have a very blurry conception of evil.
And also often to be the same people who reject the very idea of there being any transcendent standards of right or wrong.
\Frank: you brought up a good point that both points used against existence are contrary one to the other. If they hold no objective metaphysical structure of the world exists, then there is no objective evil, and so the problem of evil does not exist.
In any case, the first cause idea was already used by Aristotle himself. People for some reason think it began with the Rambam and Aquinas based on Aristotle's' idea of a First Mover. But in fact Aristotle does bring the idea of the first cause also in his Physics.
Here is the idea of the first cause in my own words:
Belief in God is rational.
Everything has a cause. So unless there is a first cause, then you would have an infinite regress. And then nothing could exist. Therefore there must be a first cause. Therefore God, the first cause, exists. QED. (You could prove the first step a priori that everything has a cause by noting that nothing can come from nothing. This makes it a priori, not just an empirical observation).