""I don't believe in God because there is so much evil in the world." Many atheists consider the problem of evil an airtight proof that God does not exist."
Not me. I think it's pretty good, but not air tight. What I do think it does is put one hell of a kink in your "omnibenevolent" deity bullshit.
"They often say something like: "I know there is no God because if He existed, He never would have let Hitler murder six million Jews.""
Which is perfectly reasonable to assume, given a god who is supposed to not only love us all but be completely omnipotent and sovereign over the universe and is also supposedly benevolent according to his followers. Once again Christianity fails the test of reality.
Now then, you have a few options: your god is benevolent but not omnipotent and couldn't stop it; your god is omnipotent but not benevolent; your god is neither benevolent or omnipotent or your god doesn't exist. None of those fit with Christian mythology though.
"A good approach to an argument like this is to say something to this effect: "Since you brought up this issue, the burden lies on you to prove that evil actually exists in the world."
Well, that's not quite how the burden of proof works but fine: Hitler exterminating ~9-11 million people and starting a war that cost another 50-70 million lives, two thirds of which were civilians. Tell me that's not "evil" by any definition of the word.
"So let me ask you: by what criteria do you judge some things to be evil and other things not to be evil?"
Let's see, stealing a loaf of bread to feed your starving family: not evil. Starting a war and exterminating fellow human beings because you happen to think you're better than them in some arbitrary way: evil. You really need someone to point shit like that out to you?
"By what process do you distinguish evil from good?"
By the process of using my faculties of reason and logic. Certainly I don't use some two thousand year old paper weight like you do to determine such things.
"The atheist may hedge and say: "I just know that some things are evil. It's obvious." Don't accept such an evasive answer. Insist that he tell you how he knows that some things are evil. He must be forced to face the illogical foundation of his belief system."
Illogical foundations? You should check your own belief system buddy.
"After he struggles with this a few moments, point out to him that it is impossible to distinguish evil from good unless one has an infinite reference point which is absolutely good."
Bullshit. If someone anonymously gives me a million dollars with no strings attached that's good. If someone tries to kill me for no reason other than they feel like it that's bad.
You're a special kind of idiot, aren't you?
"Otherwise one is like a boat at sea on a cloudy night without a compass (i.e., there would be no way to distinguish north from south without the absolute reference point of the compass needle)."
Either you saw the sun set or you will see it rise. There you go, problem solved.
"The infinite reference point for distinguishing good from evil can only be found in the person of God, for God alone can exhaust the definition of "absolutely good.""
If your god is "absolutely good" then the problem of evil wouldn't exist. If your god is "absolutely good" I want no part of him on moral grounds.
"If God does not exist, then there are no moral absolutes by which one has the right to judge something (or someone) as being evil."
Moral absolutes don't exist outside of, possibly, some universally condemned practices. Even then you'll find someone, or a group of someones, somewhere practicing it. Cannibalism is an example. It's nearly universally thought to be morally "bad" to eat your own kind yet cannibalistic tribes have been found.
"More specifically, if God does not exist, there is no ultimate basis to judge the crimes of Hitler."
Why would you need an "ultimate basis" to say that what Hitler did was wrong?
Honestly, people like you scare me.
"Seen in this light, the reality of evil actually requires the existence of God, rather than disproving it."
It does no such thing. You've yet to answer the question posed by the problem of evil.