Halloween was a Celtic and Gaelic festival which would mark the end of the summer harvest and the beginning of the winter. The "darker" half of the year has begun, the frigid season of death. The pagan Celts believed that the dead spirits visited them on this day, and so they gave them an offering of food as an appeasement, so that they may not incur their curse during this season.
In the past few centuries, people began dressing up as these dead spirits to pay homage to them.
And so when you see a slutty Halloween outfit on your timeline, know that this is just a senseless and ignorant person - a "dead" person; dead in spirit, dead in their heart, paying homage to the darkness within themselves, toiling after the fleeting frills of this world, in need of spiritual resuscitation. Whether they know it or not, they are imitating demon spirits whom they love and fear. "Surely, they had taken the devils as masters instead of Allah while they thought that they were guided." (7:30)
We have no reverence and no fear of the dead. We seek protection in Allah and no one else.
"But no one believes this anymore, all that's left are these symbols". Yes, and symbols are powerful, and thus we must not internalize symbols that have their lineage in hell.
So while people imitate the dead - both the physically dead (zombies, ghouls, skeletons, grim reapers), and the spiritually dead (materialists) - remember that Allah brings life. A person may be heedless today, but when Allah gives the gift of guidance, he will awake to his responsibilities, and be resurrected in faith.