??? I don't know. I've heard ideas about it, but none I would say are concretely historical in their basis. Most commonly, I've been told that the "trick" is something entertaining one must do in order to receive some sweets. Generally, no one requires a "trick" from those coming to their door for treats anymore and so the cry of "Trick or treat" from children is just an empty thing they utter as they walk up to a house for some candy. Actually, in my area, the kids don't bother even to say this. They just walk around in costume, collecting candy without calling out anything.
In light of this, it's very...odd to assign some nefarious, evil meaning to the "Trick or treat!" cry. Mostly, it's a meaningless thing children say to alert homeowners that they are approaching their door for candy. Whatever darker meaning it may have had to others in the past, it carries this meaning no longer - at least, not for the average kid out for some free sugar. So, why fuss about it, then? I have no idea.