Without duality there would be no unity. Both mutually presuppose each other. You can perceive the
dialectic either as a power struggle or a healing process. The choice is yours.
Assuming that every element has an opposite, or connected to an opposite, I see no reason why evil needs to be posited in order to give meaning to good. Good in its various degrees can derive just as much meaning as it stands in opposition to bad. And although there are various degrees of bad, there doesn't appear to be any need for them. To give meaning to "very good" of "kind of good" there's no need for "very bad" or "kind of bad" to exist. Any degree of good takes on meaning from its position on the Continuum of Good without needing to consider its direct relationship to bad.
So what is bad? Not to be hackneyed, but it's "
not good in any manner or degree."
(Random House Websters College Dictionary) And while this seems hardly a decent definition---defining something by what it's not---this is exactly what it is, and conforms nicely to your unity of opposites. But just how bad does bad have to be to give meaning to good? Actually, not bad at all. Lousy, poor, unacceptable, crummy, awful, gross, inferior, cheesy, crappy, and icky, all non-good things, are all sufficient to convey the notion of "bad." So, in as much as we have a good working definition of bad and have shown how relatively little is enough to give meaning to "good" is there any reason to raise bad to the level of evil
"wicked, immoral, sinful, foul, vile, corrupt, iniquitous, depraved, reprobate, villainous, nefarious, vicious, malicious; malevolent, sinister, demonic, devilish, diabolical, fiendish, monstrous, despicable, atrocious, heinous, odious, contemptible, horrible, and execrable;"
Of course not. We could all live without these beastly behaviors and still enjoy the good in life. . . . . . . along with a bit of the gross, cheesy, and icky stuff.