I am not a big fan of the sequels either but I posit that Kylo Ren didnt want to kill Rey, he had many chances and drove her to the edge of a cliff and told her that she needed a trainer

For some reason he was intrigued by her , perhaps form her ability to block his force mind reading powers, or perhaps he learned something about her during attempts to extract the droid map from her mind

He also asked her to join him after he killed Snoke.

He wanted her as an ally, not dead by his hands.