- Tapping into Rey’s personal darkness - she wouldn’t be killing Palps for selfish or evil reasons, she’d be killing him to save her friends, despite the cost to herself. That’s, like, prime Jedi material, right there. At that point, for those reasons, it’s selfish if she doesn’t immediately decapitate him.
Killing him wouldn’t save her friends. They have already been ordered to deploy, and killing the guy in charge wouldn’t change anything. Also, by your logic, Anakin was justified in trying to save his wife from death by killing the Jedi.
The difference here is that Rey has no idea that Palpatine would gain full control over her body. She was never told anything about essence transfer. The only thing Palpy has revealed to her is that “you and I will be one”, which is a lie by Palps. By that logic, Rey might assume that she has some autonomy. Whereas with Kylo, Palp’s tells him “Kill me, and my spirit will pass into you” which is a clearer description of possession.