Ash Williams and his girlfriend Linda take a romantic vacation to a seemingly abandoned cabin in the woods. While in the cabin, Ash plays a tape of an archaeology Professor Knowby, the cabin's previous inhabitant, reciting passages from the Necronomicon Ex-Mortis (or Book of the Dead), which he has discovered during an archaeological dig. The recorded incantation unleashes an evil force that kills and later possesses Linda. Ash is then forced to decapitate his girlfriend with a shovel in self-defense.
After he buries Linda, a spirit is seen hunting for Ash. Ash becomes briefly possessed by the demon, but when day breaks the spirit is gone, and Ash returns to normal. Ash finds little chance of safety, however, as the bridge leading to the cabin is destroyed. Linda's revived head attacks Ash by biting his hand. Ash brings Linda's severed head to the shed, where her headless body attacks him with a chainsaw. Ash gains the upper hand and slashes the relentless zombified Linda to death, killing her a second and final time. Later, Ash is forced to sever his bitten right hand, which has become possessed.
While Ash deals with this force, the professor's daughter, Annie, and her research partner, Ed Getley, return from the dig with more pages of the Necronomicon in tow, only to find the destroyed bridge. They enlist the help of Jake and Bobby Joe to guide them along an alternate trail to the cabin. The four of them find an embattled Ash, who is, seemingly, slowly being driven insane due to his encounter with the demon, such as hallucinating that the room comes to life and begins laughing hysterically at him.
At first, he is mistaken for a murderer by the four people because he shoots at them through the door (mistaking them as the Evil Force), but they find out the truth after listening to a recording of Annie's father, Professor Knowby, that talked about how his wife Henrietta was possessed and buried in the cabin's cellar rather than dismembered. Ed is possessed and is soon dismembered by an axe wielding Ash. Bobby Joe tries to escape but is attacked by the demon trees and dragged to her death. Annie translates two of the pages before Jake turns on them and throws the pages into the cellar, holding them at gunpoint to force them to go look for Bobby Joe. Ash is possessed once again and turns on his remaining companions, incapacitating Jake. Annie retreats to the cabin and accidentally stabs Jake (mistaking him for the demon) and drags him to the cellar door, where he is killed by Henrietta in a bloodbath. Ash tries to kill Annie, but returns to normal when he sees Linda's necklace, reminding him of her.
Ash, with Annie's help, modifies the chainsaw and attaches it to where his right hand had been. Ash eventually finds the missing pages of the Necronomicon and kills Henrietta, who has turned into a long-necked monster. After Ash kills Henrietta, Annie chants an incantation that sends the evil force back to its origin. The incantation opens up a whirling temporal portal which not only draws in the evil force, but nearby trees, the Oldsmobile, and Ash himself. Ash's possessed hand stabs and kills Annie.
Ash and his Oldsmobile land in what appears to be the Crusader-held Middle East in the year 1300 AD. He is then confronted by a group of knights who initially mistake him for a deadite, but they are quickly distracted when a real one shows up. Ash blasts the harpy-like deadite with his shotgun and is hailed as a hero who has come to save the realm, at which point he breaks down and screams in anguish.

Question: Why does Ash have to remove his severed hand?
Answer: It became possessed.
Alternative answer: Because it is possessed with demons.