It's one of those films that keeps you hooked, even though you know that things are likely to end badly.
It seems to be a modern reworking of Chaucer's Pardoner's Tale (from The Canterbury Tales) where three men come across an old man who tells them about a cache of gold hidden under a tree but warns them of the consequences of their find: they will all die.
Sure enough they end up betraying and murdering each other -- and we find that the old man who warned them was Death himself.
The plot of A Simple Plan diverges in some respects, but it's pretty similar.