This is perhaps the most commonly asked question about the film; exactly why does Hobbes choose to do what he does, kill Jonesy (
John Goodman) and then poison himself.
The answer to this question is provided in the film itself (although how valid a reason it may be in a real-world logical sense is open to debate). Early on in the movie, Hobbes mentions that in spirit form, Azazel can travel only 500 cubits, after which time, he will die if he doesn't find another host. Gretta (
Embeth Davidtz) explains that a cubit is the distance between the fingers and the elbow of an average sized person, and she reasons that 500 cubits is about 1/6 of a mile (actually, it's nearer 1/7; as 500 cubits is roughly 760 feet; a mile is 5280 feet, so 1/6 of a mile is 880 feet, whereas 1/7 is 754 feet).
With this in mind then, the question is why does Hobbes not incapacitate Jonesy/Azazel, and drive over 500 cubits away, leaving Jonesy/Azazel to die slowly, at which point, Azazel would be expelled from Jonesy, Hobbes would be gone, and Azazel would have no new host to enter (disregarding the cat of course).
The main argument as to exactly why Hobbes formulates his plan as he does is that Hobbes couldn't be sure he'd make it far enough if he ran. As he says himself in the film, "
If Jonesy dies too fast, as powerful as you are, I might never get away from you." If Hobbes ran, it could be possible for Azazel to speed up Jonesy's death, meaning Hobbes would not be guaranteed to get to the minimum safe distance before Azazel reached him. As such, Hobbes reasons that to ensure Azazel has no viable host, both he and Jonesy must die, and so he keeps Jonesy/Azazel talking so as to give himself enough time to poison himself.
Which raises the second question. After mortally wounding Jonesy/Azazel, why doesn't Hobbes just shoot himself? The end result would be the same; Azazel would be expelled, and both Jonesy and Hobbes would be dead. As such why did Hobbes choose poison? The main answer to this question seems to be that Hobbes needed to ensure that he was still alive to kill Jonesy, but also ensure that Azazel couldn't do anything to prevent him committing suicide, ie he needed to 'kill' himself prior to killing Jonesy, but still actually be alive. Obviously, Hobbes couldn't shoot himself prior to killing Jonesy, as he would have no way of ensuring Jonesy/Azazel died. On the other hand however, if Hobbes shot Jonesy and then tried to shoot himself, Azazel could have gotten inside of him and prevented his suicide. As such, by poisoning himself he manages to ensure his own death prior to killing Jonesy; Azazel can do nothing to prevent Hobbes' suicide even after he has entered him upon Jonesy's death.
It is also worth nothing that several fans have pointed out that Hobbes may have
wanted to die, hence why he doesn't try to escape after wounding Jonesy. His brother was dead, his career in ruins, his nephew gone; if he returning to civilization, he would most likely have spent the rest of his life in jail, or been executed anyway. So, perhaps, he felt that his life was over in any case hence he could make his death serve a purpose.