The reason animal masks were chosen as a disguise is because the Animal Mask killings in the Hotline Miami
games from the '80s and '90s have become ingrained in American culture. Everyone knows about the animal masks, campfire stories about them are traded among children, and Halloween costumes of them are popular even.
The easiest way to have everyone assume that it was just lunatics killing a family would be to appropriate that mythos.
That, or her Crazy Survivalist
dad showed her Home Alone
a lot growing up. Either way, it explains why she'd think to Booby Trap
the house so thoroughly.
Erin was sent a mental institution following the movie's ending
Following an investigation of the movie's events, where the police found the masked men and the proof one the phones. Although she could've been declared innocent via self-defense, Erin decided to take a plea bargain and was institutionalized to help her cope with what happened.