Think about what you just wrote, say it aloud, and then question if that really makes any sense at all. If you think it does, then say it aloud again, and again, and again.

Now, what you just suggested is basically the argument of a means to an end. Fairly self-explanatory, but also logically flawed, simply because it limits the power of things. If evil exists simply because it is a means to an end, and we understand that, then it's need is no longer needed. The end is understood and theoretically could be achieved through that knowledge. Now, again, perception comes into play. One doesn't truly "know" anything unless it's experienced and yaddah, yaddah, yaddah...so say empiricists. But then you have the problem of whether or not it is possible to achieve an end without the means. Theorectically, yes. It's possible to have a man lead a purely good and wholesome life, althought with the comprehension of evil, and can still achieve the same end, but theorectically without the means.

And then you have the problem that basically coincides with the Correalation of Opposites. That to truly understand "good" one must have "evil." And yet, we can understand "thing-x" without having "thing-y." For example, gravity. Us as humans can have an understanding of gravity without having its opposite. Thus, does one truly need evil to understand good? Logically, no.