Been a long time since I played it. That would explain why Jin believed he can destroy the devil by killing Azazel. But since it didnt work I guess he was wrong.
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Azula: My mommy didn't love me so I'm going to burn down your village.
I did have a theory, but it was a long time ago...
Anyways, I suppose the general gist of it would be that the Devils themselves use the "Devil gene" to make a human host more capable of accepting their presence within the body, hence why Kazuya had to make the deal with Devil originally.
Now, suppose that these "Devil genes" can be passed down to succesive generations in order to provide an immortal Devil spirit have unlimited hosts?
The Devil that posessed Jinpachi was a seperate entity than the one Kazuya made a deal with, but both where originally spawned by Azazel, along with however many other Devil entities he created. Now, Azazel is immortal, and strong enough to exist as a physical entity without a host, the other Devils that it spawned are probably not so powerful. I would suspect that the Devil that Kazuya made a pact with, and the one Jinpachi was posessed by are among the stronger spawns though.
Now, there are 2 other theories I have reguarding this. I do not think that Kazuya's Devil has actually been weakened, rather I think that Devil Jin is merely a spawn of Kazuya's devil given it's own life, rather than it bein "Split" from the original entity. I say this because there was no actual alternate personality with Devil Jin, it was more like Jin himself going crazy, unlike Kazuya's transformed states where a new personality emerged entirely (reffers to Tekken 4's showings of how Devil had it's own personality that was talking to Heihachi). Now I'm well aware of what Devil says about Jin being his other half... but that could simply be reffering to either Jin having a comparable amount of power, or it could even be reffering to the Angel aspect, due to the Kazama bloodlines capability of neutralising the Devil gene's powers/potency.
The other theory is that perhaps Jinpachi's was supposed to be the second "Evil Star" refferenced in Azazel's prophecy. I say this because when Jinpachi was revealed to have a full Devil posession like Kazuya's originally was, that would have made the prophecy true of inpachi had held on long enough to fill the world with negative energy. That may have been where he was refferd to have "Wipe out all life on earth" kind of power from, it may have been refferencing Azazel's resurrection...
However, these are simply suppositions on my part, not fact or anything, still, I would like to hear oppinions on this.
Ok well... The way i see it, basically jinpachi's feats are obscure, if not unknown, and we cannot make a cannon claim on what he can or can't do... So if you want to go by ''feats'' i'd say akuma... But if we ever see more of jinpachi's (who seems pretty bad ass as of his intro.) feats, then it's still ''up in the air'' sorto speak..