I heard a hunting story once where it was claimed the deer was spooked, ran right into a tree and broke its neck without having to be shot. The next easiest way to go is probably freezing to death, like not from starving but falling in a stream or something. Its hard to tell how they process pain. Other than that, I think the wild would tend to be more brutal than an ethical kill. They will try to keep going through whatever they face, so anything that just overwhelms them is about as good as can be expected.
Domesticated animals can have an easier death, but on the other hand the living conditions of bacon you buy from a supermarket could have been pretty inhumane. There is no easy way to die, and I kind of resent people's problems with killing to the extent that it detracts from living.
