[Nilpaurion Felagund]

Iarwain got to this first, but I would also like to share my disagreement. Here are my reasons.

1. Evil did not co-exist with good in the beginning. No-one was evil from the beginning, after all.

2. Evil is but a corruption of good.

First, of course, good would have to be defined in the context of Arda.
What is good? It is the fact that creation - the ability to create and the fruits of the ability - belongs to Eru alone, and that the Children of Ilúvatar deserve life.
So, in this context, how did evil begin? Melkor desired to be creator himself, and when he couldn’t do so, he defaced creation instead.

It is said that Morgoth’s worst evil was the creation of Orcs. Why? Because he corrupted the works of Eru both on the outside and on the inside. By inducing this race to hate, to disrespect others’ right to live, he removed the Orcs’ right to live.

I know this seems wrong, but have you noticed that nobody has called a truce with an Orc? It is because they’d either kill you anyway if you sue for peace, or that they wouldn’t understand your mercy and fight to the death.

3. Good and evil could not co-exist forever. They are both locked in a battle for survival. The history of Middle-earth is a testimony to that. In the end, one would have to be utterly destroyed, and the other would rule. Considering the history of Middle-earth, and Eru himself, we could see the outcome of such a battle.

So, in conclusion: Evil did not share its beginnings with good, and it would not share its end. There is no balance of good and evil in Arda, except maybe that evil should be non-existent.

Oh, Melkor. You naughty, wayward child.