Bad Taste

The Astro Investigation and Defence Service (AIDS) sends Derek, Frank, Ozzy, and Barry to investigate the disappearance of everyone in the town of Kaihoro, New Zealand. They find the town has been overrun by space aliens disguised as humans in blue shirts. Barry kills one of the aliens and is attacked by others. After Derek notifies Frank and Ozzy, he begins torturing Robert, an alien they caught earlier. Robert's screaming attracts a number of aliens in the area. Derek kills the would-be rescuers, but he is attacked by Robert and falls over a cliff, to his presumed death.
Meanwhile, a charity collector named Giles is passing through Kaihoro. He is attacked by Robert, who has been eating the brains of the alien killed earlier by Barry. Giles escapes in his car and stops at a nearby house for help. Another alien answers the door and captures Giles. He later wakes up in a tub of water and is told he is about to be eaten. Derek also wakes up to find that he landed in a seagull's nest. He also finds that his brain is leaking out the back of his head, so he stuffs it back in and uses a hat to hold it in place.
That night, Frank, Ozzy, and Barry sneak into the aliens' house and find a room filled with bloody cardboard boxes. They kill an alien by ripping off its head and Frank wears its shirt to infiltrate an alien meeting. He finds out that the residents of Kaihoro have been harvested for alien fast food. Robert vomits into a bowl, which the aliens dine on, including the disguised (and disgusted) Frank. He escapes and tells the team members of the plan. They sneak out to save Giles as the aliens sleep.
At sunrise, they try to leave but are attacked by the aliens. Derek's hat is shot off, and he starts losing more of his brain, so he uses his belt as a headband. He grabs a chainsaw from the boot of his car and heads for the alien house. As the boys leave with Giles, the alien leader (Lord Crumb) and his followers transform into their true form and follow. Ozzy uses a rocket launcher to blow up Frank's car, which has been overrun by aliens.
Frank and Ozzy hunt for Lord Crumb and kill a lot of aliens along the way. Meanwhile, Derek kills an alien with his chainsaw and replaces the missing parts of his brain with alien brain. An alien prepares to shoot Frank and Ozzy, but it is beheaded by Derek after he bursts through the wall behind it. Frank and Ozzy are shocked to see him alive.
As they run, Lord Crumb shoots Ozzy in the leg and Frank fires his rocket launcher at the leader, but it misses and almost hits Derek, finally taking out a sheep in a nearby meadow. Derek is knocked out by the alien leader and the house transforms into a giant space ship, which blasts off into space with Derek still aboard.
On board, Derek looks out the window to see that he is leaving Earth. Lord Crumb hears a strange, loud noise and investigates. He is killed by Derek, who ambushes him and cuts the alien in half with his chainsaw. Derek proclaims into his phone: "I'm born again! I'm coming to get you bastards!" He then puts on the alien leader's skin, laughing maniacally as he rockets towards the alien planet. On Earth, the guys drive away into the sunset in Derek's car.

Question: Who beheads the alien?
Answer: Derek