You're Next

The film opens with a couple being murdered in their house by assailants wearing various animal masks.
Erin accompanies her boyfriend Crispian Davison to his family reunion at their rural Missouri vacation home, adjacent to the property of the murdered couple. Present are Crispian's parents Aubrey and Paul, his older brother Drake and Drake's wife Kelly, his younger siblings Felix and Aimee, Felix's girlfriend Zee, and Aimee's boyfriend Tariq. During an argument at the dinner table between Drake and Crispian, Tariq and Drake are shot by crossbow bolts from outside the house, Tariq fatally and Drake moderately wounded. With their cell phone signals jammed by the attackers, Aimee runs out the front door to get help, but runs into a garrote wire which slashes her throat. Crispian leaves the house to find help. Aubrey is brought upstairs to her bedroom by Paul; when Paul leaves the room, Fox Mask appears from under the bed and kills Aubrey. The rest of the family rushes upstairs to find Aubrey dead with the words "You're next" scrawled on the wall in blood.
Erin begins securing the house and texts 911. She encounters Tiger Mask and evades his attack, wounding him in the process. Kelly returns to the bedroom and discovers Fox Mask still hiding under the bed. She panics and runs to the neighboring home. After pounding on the window for help, she gets inside but discovers the murdered couple and is soon killed by a maul-wielding Lamb Mask. Back at the house, Tiger Mask once again fails to murder Erin and is in turn killed by her with a meat tenderizer; it is later revealed that Tiger Mask is actually Lamb Mask's brother. Lamb Mask finds Drake, but Erin stabs him with a screwdriver and he retreats.
While exploring the house, Paul finds evidence that the killers had been staying in the house for a while. He tries to tell Zee and Felix, but Fox Mask appears from behind and slashes his throat. It is revealed that Felix and Zee hired the assassins to murder the family and collect their inheritance. Felix lures Drake to the basement and stabs his brother. Meanwhile, Erin and Zee set up traps together and Erin explains that she grew up on a survivalist compound where she learned her combat and survival skills. Zee is about to attempt to kill Erin, but is interrupted. Erin overhears an argument between Felix, Zee, Fox Mask, and Lamb Mask, and takes the chance to escape from the house. Lamb Mask pursues Erin but she manages to kill him. Erin returns to the house and sets up a trap at the front door. Seeing Fox Mask enter through a window, Erin sets up an ambush in the basement and kills Fox Mask. With their hired assassins dead, Zee and Felix enter the house to kill Erin themselves but she manages to brutally kill them both. Crispian calls Felix's phone but inadvertently reveals his involvement in the scheme to Erin, not realizing Erin has killed Felix and taken his phone. Crispian returns to the house and, ignoring his attempts to bribe her with money, Erin kills him out of anger.
A policeman arrives and shoots Erin in the shoulder, having seen her kill Crispian. After calling for backup, he attempts to enter the home but is accidentally killed by Erin's ingenious trap at the front door as she yells to warn him. The credits imply that Erin is questioned by the police as a suspect in the killings.

Question: Who killed Fox Mask?
Answer: Erin