A passage from the Star Wars: The Last Jedi novelization reveals Rey considered murdering an unconscious Kylo Ren following their altercation in Supreme Leader Snoke’s throne room.
With this decision, Rey was avoiding the same error Luke made in thinking Kylo’s future was already set in stone.
The Force does not belong to the Jedi, Rey, or anyone else who uses its power.
When Rey saw Kylo was still alive after the lightsaber split, she realized he still had a part to play in this story.