Regardless of the effort that an individual may place into commercializing this holiday, it cannot be denied that Halloween has pagan roots. It is of Celtic origin. It was observed in Ireland and Scotland, and was adopted by the Church of Rome in the fifth century. Halloween contains occult elements, from sorcery to witchcraft to demonism. Such concepts are condemned in Scripture (Leviticus 19:31; 20:6; Deuteronomy 18:9-12; 2 Chronicles 33:6; Galatians 5:19-21). Partaking in occult activities is idolatry, which can range from Ouija boards to tarot cards to fortune telling and so on.
People cannot, however, utterly disassociate themselves from Halloween, as it is another day of the week. God is our Creator and transcends time. Everything rightly belongs to Him (Psalm 24:1; 1 Corinthians 10:26). That includes Halloween. We should be in prayer on a daily basis. We should be glorifying Him on a daily basis. There are holy ways of going about matters in this life. There are also sinful and unwise ways of addressing issues. Moreover, anything can be abused and misused.
The act of dressing up as a cowboy and going door to door with the intention of receiving candy is not in and of itself sinful. To illustrate the point, the Apostle Paul declared that eating meats offered to idols is morally permissible, provided that fellow brethren are not offended (1 Corinthians 10:25-33). If eating causes one to "participate" in the sacrifice itself, then why would the Apostle Paul tell his audience that it is fine for them eat the meat offered to idols? In the same manner, whether or not a person goes trick-or-treating is a matter of conscience. No homage is given to false gods.
A person whose conscience is violated by such a cultural tradition has every right to express disagreement. Yet, we should strive to not become stumbling blocks to each other. This can serve as a lesson in Christian love, fellowship, discipline, and humility. Paul said, "One person regards one day above another, another regards every day alike. Each person must be fully convinced in his own mind...Therefore let us not judge one another anymore, but rather determine this—not to put an obstacle or a stumbling block in a brother’s way" (Romans 14:5; 13). If a Christian does embrace the sinful themes of Halloween, then he or she is indeed guilty of worldly compromise and needs to repent.