Mother Knows Best
In Billy's character analysis, we briefly speculated where Billy's bravery comes from. The answer is simple: it comes from his mom.
Because Billy's mom is like the love child of Martha Stewart and Rambo.
Even Linda Hamilton in Terminator 2 would stay away from Billy's mom, especially if a microwave was nearby. When the gremlins mutate, Billy's mom wipes their scaly butts off the map without hesitation. One is a blender. Bam, he's juice. One is near the microwave. Ding, that thing is never being cleaned. Another throws things at mom. She slices, dices, and juliennes him with a carving knife. Mom: 3 Gremlins: 0
Unfortunately, a gremlin in a Christmas tree gets the jump on mom, once again showing us how Christmas ruins everything. Billy arrives just in time to save mom and whisks her to safety. He should have brought her with him. She'd have turned that chainsaw on Stripe and shown him the true meaning of buzz cut.
Mom isn't 100% guerilla warrior. She has a softer side. She lets the family's financial troubles roll off her back, and she lets Rand think his inventions actually work. The woman is a freaking saint.
But did you ever consider that she's been repressing anger toward her feckless husband? The gremlin attack is just what she needs to vent her rage.