I also grew up without Halloween, both because of religious reasons (“It’s evil”) and even the more innocent practices simply didn’t translate, culturally (“Why do you need to beg the neighbours for sweets?”).
Halloween’s origins are from pre-Christian, Celtic/Anglo times, so originally there was no connection with Satan whatsoever. When the isles were Christianised, Halloween became a pre-fest to All Souls Day, with some of the old pagan customs remaining. In the UK, until the last few generations, people used to go door to door, exchanging “soul” cakes and money as a way to honour the dead. Nowadays it’s become re-imported as a meaningless and commercialised holiday.
So, to answer your question – is trick or treating “bad”?
It’s clear that most trick-or-treaters look for nothing more than a good time.
But that’s not enough, IMO, to say celebrating Halloween is okay. You note one major problem: Halloween is still a holiday for Celtic Neo-Pagans. This current connection is, for me, too close for comfort.
So while you should reconsider going trick-or-treating, and although I can’t decide for you, your mother has. You’re obliged to respect her judgement.