Otdfbt

Qui-Gon Jinn has finally realized he is an idiot and that everything is his fault. Everything that has happened (fall of the old Jedi order, rise of the empire, the rebellion, the First Order, etc.) can be traced back to his decision to train Anakin Skywalker. If he had just listened to the green troll, none of this would have happened and the galaxy would be at peace. Realizing that he is the retardant-supreme of the series, he has set upon an ambitious plan to correct his past mistakes.

The force is pure energy, which runs through and is made up of all living things. When a person dies, they become one with it; their consciousness being subsumed by its presence. There is no past, present, or future in the force, as time is not truly linear. A strong enough force-ghost who has retained their consciousness can therefore appear and act upon the material world.

Qui-Gon has decided to go back in time to before he made that decision about the brat. Instead of training him after meeting him on Tatooine, he plans to kill him, his remaining family, as well as anyone who traveled with him to the planet. To do that, he must take over and possess the bodies of living victims in order to get there. Unfortunately, he soon realizes that possessing a individual leads to its inevitable decay. The body begins to rot not long after possession, despite the fact that it is still living. This forces him to body-hop from person to person, leaving behind desiccated, withered corpses in an attempt to stay ahead of time.

Why would something like this happen?

When a spirit possesses a body, it controls its movements: legs walk, hands grasp, eyes blink at the whim of the possessor. Each movement is a conscious act.

What most people forget is that the body has completely lost control of everything, so it is up to the spirit to keep the heart pumping, the lungs breathing, and sphincters closed.

Besides the fact that the spirit is probably unused to these things due to being not alive for so long, we generally suck at continuous body control. Don't believe me just because I said, experiment with it. Find some friends and say to each one: "you are breathing on manual now". Some will immediately take a deep breath and breath awkwardly for a few seconds as they become conscious of their own lung movements. By the way, you are breathing on manual now. Don't worry if you are affected, it resets in very short time.

Back to possessed bodies without a blood flow - anoxia starts after several minutes of the blood not refreshing oxygen for the brain. Some quick googling shows the amount of time needed for its onset varying, but it is usually between four to six minutes. It may vary with general health conditions, and in a Star Wars setting may also vary by species. After ten minutes you may expect permanent brain damage, and after thirty you might be quite sure of brain death. Other tissues die as well due to lack of oxygen.

The corpse may be seem fresh for a few hours. If you want it to stink after a few minutes of possession just say that midi-chlorians accelerate the rotting process. That is even cannon for the series, seeing as they did exactly that with Episode 1.

Perspective shift: the body is not alive.

Once a body is possessed, it dies. It remains a flesh and bone puppet to the possessing entity. The entity can mimic life signs such as breathing and heartbeat but it's not much different to applying electric stimuli to the correct parts of a cadaver - it only appears living.

So, a dead body will naturally start decaying.

There are few other problems the possessed cadaver might face. No wounds would heal, for example. Nicks and cuts and other wounds would keep leaving scars on the body and might force the possession to end early, as it becomes increasingly conspicuous.

Another problem is warm temperature - you would need to keep the body reasonably cold to prolong the time before it becomes unusable. Maintaining the humidity and other environmental factors might mean the possessed body could be kept operational for a very long time.

To maintain the cover, an old person can be a good target. First of all, people don't pay too much attention to the elderly. An old person who lives alone with no other family and not much contact with others for a long time is something people will notice but not immediately think "it's a possessed body". And old people are given a large berth for their eccentricity.

So what if the older gentleman lives alone and doesn't exit his home? And he prefers his home very well r̶e̶f̶r̶i̶g̶e̶r̶a̶t̶e̶d air conditioned? It can draw comments, sure, but is within what society is willing to turn a blind eye to. Maybe the older eccentric gentleman is a doctor from a foreign country, too. At this point all the strangeness can be easily accepted.

Until one night the cooling system fails and the dead body falls apart by the time it could be repaired.

Post contains allusions to a short horror story that you can read here. It kind of spoils the twist but...then again, the twist can be seen coming from very far away.

What you describe might generally be called "localized necrosis." As suggested in comments, gangrene is one form of this. Something similar happens with certain spider bites (though it's often possible to avoid infection in that case because of the small affected area), in Hansen's Disease (colloquially known as leprosy), and necrotizing fascitis ("flesh eating bacteria").

This happens when a limited portion of the body dies while still attached to the living flesh. If unchecked, and in a form that spreads, it's usually fatal. In some cases, (where the dead flesh can be excised) it may only lead to amputation or scarring.

Why would this happen when force-possessing a living body?

Qui-Gonn's force isn't complete (because he's dead). Some of what was once the essence of Qui-Gonn has taken a wrong turn in the essence of the Force -- and if the body is partially animated by a force ghost, while some fraction isn't so managed, and the original owner's "ghost" has been forced out, the parts that aren't under direct control start to die.

In general, the idea that when the spirit of the dead possesses a body, the body starts to decay, is so narratively consistent, you do not need any explanation at all.

The setting being the Star Wars is an additional factor that simplifies matters. As far as the force users are concerned, it strongly runs on 'good is beautiful, evil is ugly' principle. Using the Dark side makes force users get saggy pale skin, red eyes and so on. Necromantic possession seems very much a Dark side power, so there needs to be no other explanation that the Dark side corrupts the possessed bodies, burns the life out of them and generally makes everything messy, ugly and stinky. Possibly, more detailed explanation could be that the bodies lack biological mechanism to channel the force from outside (if you believe in midichlorians), so they run for some time on the inherent force every living being has, but they burn out, especially if the possessing spirit tries pushing them to use any force abilities. Force is life, if you loose force, you get dead - as simple as that.

UPD: by that logic it means that if your force spirit manages to capture a force user body, it may go all Palpatine-ugly on him, but won't decay.

If your setting is not Star Wars, but it was merely an example, other explanations proposed here work nicely too.

The force ghost does not feel any pain

As such, the host effectively caches a non-congenital version of Congenital Insensitivity to Pain, a horrible real-world disease whose sufferers tend not to live very long.

It turns out pain is important for continued survival. Most people have experienced forgetting they have keys in their back pocket, sitting on them against a flat surface, felling pain, and immediately getting up. Well, if there's no pain, they tend to not immediately get up, instead leaving their full weight on the keys for an hour or three and really hurting themselves.

There's a million such hazards for those who feel no pain. The ones who live the longest do so through discipline and awareness.

The force ghost can go arbitrarily long without even knowing what the issue is. Body parts start to necrosate without warning, and eventually the host dies and the ghost has to seek a new one. Eventually there's a big epiphany.

Depending on the format, it may be nice to leave clues for the audience. E.g. in a book, they "put the keys in the back pocket". A few pages later, they "sit down on the hard wooden floor" for a long conference. A few more pages and couple days later, they notice their leg is blackened.

Every soul is meant to have only one body, and every body is only meant to have one soul.
When Qui-Gon's soul enters another body, the original soul is pushed out, meaning the death of the original person. Their soul is gone, and their body naturally begins to decay.
The Force, or the fact that Qui-Gon's soul is in the body, enables the body to move and act like a living being, but it is in reality dead.