The mischievous tricks of the Halloween pranksters had become so out of hand by the days of World War II, that in fact, the day was known as Mischief Night. And like anything and everything that honors the Devil and detracts from the glory due to God, the ugly customs of the pagan holiday of Halloween were promoted and spread and practiced in greater malice until they became what we now know them to be.
We can easily see simply by reading the history of this holiday that what had been a pagan custom was combated by the Church for an honorable and charitable reason and how the Devil has used it in a perverted manner in order to destroy, if possible, what should be a ritual of beautiful custom. We have a duty, as Catholics, to practice ONLY those rituals designed for the honor of God and the relief of the suffering souls and given to us by Holy Mother Church as a means of furthering our salvation also. To partake in the practice of pagan and devil-honoring rituals is to offend God in a most demeaning way. We should therefore strive rather to return to the beautiful customs of our forefathers and practice in its entirety and with all the purity of its original intention, the customs of All Hallow’s Eve.
Source: Catholic Insight.
I can't see how any Catholic can participate in any Halloween activity--even one so seemingly benign as trick-or-treating--without committing mortal sin.