Everyone has both Light and Darkness in them. It's natural. But, there are bad people in the worlds, and they will use Darkness in a bad way.

But, like with many things, having a balance of Light and Darkness can be helpful. Just as long as you dont use either inappropriately.

Think about it. There are plenty of people here who use Darkness who aren't bad people. In fact, many of the Riku's use both, and they're some of the most powerful I've seen, while still fighting for good.