Izo solved!
Not really... I can't claim to be the be all end all of symbolic interpretation, and clearly Miike hasn't outwardly expressed any real point but I will let you know how I see it.
The Japanese people generally have a tendency to believe in re-incarnation, as many Buddhist countries do. Izo obviously is killed in the beginning by crucifixion which is also a form of severe humiliation only intended for the most wretched types of people. From this point forward, the movie follows his soul through a series of spiritual tests. Where should such a soul go? Not Heaven, nor Nirvana as known from the beginning because he was simply a bad man. So, he can either be re-incarnated or go to hell. There is even a scene of him being reborn from a mysterious woman to show him his possibilities. He is placed through different time frames in which his re-incarnated soul could be placed if fitting and he abides. Izo has so much hatred and anger that regardless of where he is put, it only will result in more heartless killing. This is why we see him as a lawyer and again as another character (I forgot which) and he actually kills his proposed re-incarnated being. Izo will not follow anyone ever again or be who others would want him to be.
As the movie goes on, we see his fangs starting to protrude more, a bloodied mask, and red eyes which illustrate how evil he has truly become. It seems as though re-incarnation has been deemed too good and a waste on him, so he is given a chance to have his soul cast into hell. Towards the end of the movie, everyone he comes across is like a vampire, zombie, or guardians. (Bob Sapp has a very similar look to some of the guardians to the gates of Heaven and Hell in Japan, otherwise known as Tengoku and Jigoku respectively. The guardian names elude me though...) When he finally reaches the decision makers, who also resemble many famous historical figures, he would kill them too with the utmost hatred. It seems hell is also too good for this monster.
At the end, he comes across the man with the snake, who I feel is God and the Devil together. The calm and collective man wears white and is imposed against the sky in its finality and the snake is just a symbol I would believe the Japanese to know from Adam and Eve. That may go disputed I guess. As he is given the chance to be just within reach of the two to test his last second implications, he disappears. His soul has been erased.
I know some people might full heartedly disagree with me but that's how I saw it. As for the mystery woman who claims that she is not supposed to have direct contact with Izo yet does, she is a bit more confusing for me. She could be a spiritual guide of some sort, or the mother who is waiting for him on the other side if he abides by what is desired of him - compliance or humility.
I have only watched this movie once as well, so I am sure people might find other things that could contest this as well. I am eager to hear what other people think.