The ninetails is immortal, even if you kill his body, he'll reappear elsewhere...
so in order to buy time, he's sealed in Naruto...

the real question is why he's helping Naruto.. it was never stated that he'll die, he was just telling Naruto that it's courgoues of him to threat him like that (in other words, Kyubi was saying: "you say that if you die then i'll die too, ha? that's some courge you got there, brat, I like that so i'll give you some power")...


and don't say i'm wrong, becuase i'm right. deal with it