Also, if it wasn't there, G-d would have been unfaithful. He can't do that! (Or won't, wtvr)
Evil did exist before Creation, even if only by virtue of what G-d is NOT. His creation has to reflect that truth, since He decided to reveal Himself via Creation.
Now for those that get that and want more:
Lucifer fell before man arrived on the scene. Lucifer was "the cherub that covereth," and we are also told he was a light-bearing being. Yeah, but he covered what?? No, we're not told specifically; but we do know G-d is light, and he refers to evil as darkness. So, in an all-encompassing way, we can infer that Lucifer's "original estate" was to cover evil so nobody would be tempted by it.
Lucifer left said first estate, and it's rather ominous that we're not told about those details. His fallen nature and name is satan, so where does he show up first? You got it, right at the only evil in existence! The tree of the knowledge of good and evil did NOT give Adam and Eve knowledge of good, they already had that, in abundance, for all eternity. So for all practical purposes, it was the sole manifestation of evil. Instead of covering it 'out of sight out of mind' and therefore no temptation, he attempts to conceal it's evilness by making it out to be something good. That's not just satan trying to be cute, that's his fallen nature.
"We are not ignorant of his devices."
This also explains G-d's attitude toward our sin, a la the Prodigal Son. He knows what we're up against, and wants us to WIN!
If our OP ever comes back, imagine what he'll be thinking of what we did to his thread (LOL)