Jar Jar Bricks said:
You have to kill him with the dark side in order to be possessed. And thankfully the Force ghosts we added in serve a purpose beyond fan service: they hold Palpatine’s spirit in place so it’s finally destroyed.
Agree thats always been my interpretation of it, Sheev struck down Plagueis with dark intent and got possessed probably like the many other Sith before him so Plagueis needed Rey to do the same but Plagueis ended up being destroyed by his own powers in the end as the ghost Jedis shielded Rey.
