Remember it is not candidates who decide most things. It is their campaign staff. In this case, whoever hers is, they are attached to the Never Trump handful across the nation, but particularly at the RNC.
It is THEY who have told Haley that if she stays in the race, then after Trump is assassinated it will be she who will be the party’s leader. No, she won’t win this year, but she will inherit the party leadership for next time, because if they can just get Trump assassinated, then all the populace will return to proper worship of the Swamp.