He didn’t ask about hypothetical alternative candidates, they asked what principle you hold that would prevent you from voting for her - i.e, what action would Kamala Harris have to take to make you feel she was as bad as or worse than Trump? Would you support her if she nuked Iran? France? Minnesota? If she had immigrants shot? Your family? Would you still vote for Kamala Harris in the upcoming election against Donald Trump if she personally executed your family?