I'm confused with the ending, help please. (TOTAL SPOILERS)

I couldn't figure out the whole immortality thing by the end of the movie.

First the witch sticks her hand in his chest and gives Kaulder immortality.
Towards the end of the movie, she sticks her hand in his chest again and takes it back, so now Kaulder can be hurt and he's not immortal.

Then at the end, Kaulder gets severely hurt and shot multiple times, he's about to die. But he wakes up or something, and can still attack the witch. He takes her out, and he should be dying, except he's not dying. But then he looks over and sees the witch heart still beating. So he gets up and is about to destroy the heart, but the girl says, "No, you'll die!"

What? He already lost his immortality, why would he die if he kills the heart? The witch already took "the curse" back and made him mortal again. Destroying the heart shouldn't matter at all.

So at the end, Kaulder keeps the heart, so he can stay alive and keep on fighting evil. So that means he's immortal again somehow?

None of this makes sense. By the end of the movie, can he still be hurt and is he still immortal, or what? His old British buddy makes a joke about not getting younger, which implies Kaulder is getting older. So he's not immortal, so why does the heart matter at all.