I don’t know what could be added to this profundity.
The unreferenced “it” adds just that bit of unsettling to an otherwise pretty pat statement.
What was the point in stating it? The context was Paradise Lost, so perhaps my writer was pointing out that the entrance of Satan into Eden necessitated the death sentence God gave out to Adam and Eve. Following that idea up in an articulate manner might have been interesting; what the student wrote, however, did not make clear that she was trying to go in that direction.
Well, as written, a lesson in the inevitable. I wonder, though: is the mixture the thing that creates the “circle of life”? Would having only good always lead to life, and having only evil always lead to death?
Since such purity is generally considered to be reserved to heaven and hell, I suppose we won’t get a chance to test the hypothesis until the lesson is beyond our use.