Everybody has some darkness in them.
Consider how Naruto flipped out and let the Kyuubi free when Pein "killed" Hinata. The Kyuubi latched on to his negative emotion and convinced Naruto to unbind him. Same thing when he fought against Orochimaru. He didn't free the Kyuubi, but his body was under the Kyuubi's control. Naruto explicitly said the Kyuubi took over because he was so angry at Orochimaru.
This is actually a pretty well thought out plot point...