[Mr. Johnny 5]

Spoiler for manga:

I think it was revealed in the manga

Spoiler for manga possible answer to this thread:

The 4th stated that there was one controlling the Kyuubi. At the time i think that the 4th wasnt sure that it was Uchiha Madara but when he fought him there was no way to defeat him. He couldnt be killed or brought down without some sort of special power. Probably because of his own fear of the village safety he sealed it inside Naruto "his own son" to eventually fight this character hoping the Kyuubi would give him the power he needed to kill or defeat that man.

That was his reason for sealing the Kyuubi inside Naruto. He believed he could master or control it. If you ask me. The 4th simply couldn't kill the Kyuubi either. It was his only option for a mystikal beast that simply heals all the time. If you ask me the Kyuubi is no different from Majin Buu except that no one has the power that of a nuke to completely destroy his body like Buu. It is massive afterall compared to that of ninja's.