Well...it wasn't exactly that easy. Riku was really in deep and he had to find his own path. He hid from us for a while before he could accept that he was no longer a person of the dark.

[Sigh. Ansem.] He wanted to send the entire universe into the darkness, to make it brief.

The Heartless are monsters in our world. They're what come to life when a person loses their heart to the darkness.