If you swallow me, you'll live,
but if I swallow you, you'll die.
What am I?
Reason why it can't be an animal
Drink water and you live. Drown in it and you don't.
Another alternative solution:
A bitter truth
If you can swallow it, you can move on in life. If you let the bitterness consume you, you could die of heart attack or hypertension.
I think that the answer is:
If he swallows the Ghosts, he lives. But if they swallow him, he dies.
If you swallow your pride, you'll live.
But if you let pride swallow you, "pride goeth before destruction, a haughty spirit before a fall".
A vicious dog (or any suitable animal)
You can swallow an animal if you cut it up, but if it swallows (eats) you, game over.