WARNING: Japanese rant follows!
ni naru = become/will become
ni natteru/natter iru = are becoming
That's what Kenshiro actually says "Omae wa mou shinde iru", which naively translates into "you are already dying". Not so cool.
I asked a Japanese friend of mine about this, years ago, and she explained that "shinu" (to die) is some kind of special verb, where the tenses don't work quite the same way, and that, anyway, shut up, "dying" means "dead", okay? (I guess it makes sense. In English, when we say someone is "dying", they're not really dying, they're "about to die". Actual dying is instantaneous)
Normal verb - taberu (eat)
taberu = eat/will eat
tabete = to eat/the concept of eating
tabete iru = eating (right now)
tabeta = ate, in the past
tabete ita = was eating, in the past
Instantaneous(?) verb - shinu (die)
shinu = die/will die
shinde = to die/the concept of dying (Hell Girl says this "shinde miru" = "try dying?")
shinde iru = dead
shinda = died/is dead (but not just now, because that would be shinde iru)
shinde ita = was dead(?)
So anyway, the logic is that naru(become) is also an instantaneous verb, and would thus take the same form in the sentence as shinu did.
Therefore: Omae was mou ruru rokuju-san ni natte iru. I admit, it doesn't have quite the same ring as the original