It began as All Hallows Eve, the Celtic equivalent of new year's eve. It was believed that on that night, the cairns (sacred mounds, usually tombs of some sort) would open, and creatures of the Otherworld could walk freely in ours.This included both fey (fair folk, or elves, if you will) and demons. And neither were on particularly good terms with humans. You can read many stories in Celtic mythology about the harm done to humans by fey and monsters (a particularly good one is the story of Finn McCool, a man who kills a monster with an Otherworldly weapon).
My point in saying this? It shows the roots of it in paganism, and even today celebrates death and horror, neither of which are things for Christians (who have been redeemed from death and in Christ have nothing to fear) to celebrate.
Perhaps you could argue I'm being rather hypocritical, given my change of username. Maybe you'd be right. I'm more doing it for the memes than for the point of celebrating All Hallows Eve, so I'm somewhat justified.