Master of Sauron

[Legolas]

I gave this one in my previous post. Explains why elves weren't totally good.

But the Elves are not wholly good or in the right. Not so much because they had flirted with Sauron; as because with or without his assistance they were 'embalmers'. They wanted to have their cake and eat it: to live in the mortal historical Middle-earth because they had become fond of it (and perhaps because they there had the advantages of a superior caste), and so tried to stop its change and history, stop its growth, keep it as a pleasaunce, even largely a desert, where they could be 'artists' – and they were overburdened with sadness and nostalgic regret. In their way the Men of Gondor were similar: a withering people whose only 'hallows' were their tombs.

Evil comes about from good intentions:

The Enemy in successive forms is always 'naturally' concerned with sheer Domination, and so the Lord of magic and machines; but the problem: that this frightful evil can and does arise from an apparently good root, the desire to benefit the world and others - speedily and according to the benefactor's own plans - is a recurrent motive.

This quote is about Melkor, but can be applied to any evil - it comes about because the benefactor(s) [elves in this question's case] thinks it would be best for his will to be done while ignoring Eru's supreme will. This will always fail because of Eru's design - the world will continue with his will and those who oppose will naturally fall.