In this way of thinking about it, Palpatine wanted Rey dead as a child bc he sensed her power as a threat. Her parents wouldn’t betray her location so he killed them.
Also because “he saw what [she] would become” according to Kylo. So basically he knew that whenever she returned she would be a powerful user of the light. Thus why he would want her dead so Kylo doesn’t come back to the light, or possibly get killed.
EDIT: Funnily enough, both of those last things happen to Kylo technically because she was allowed to live.