Bellatrix Lestrange should have been a Hufflepuff. If the hat considered traits had more than traits valued, I believe she would have been one. Or it would have been considered and she would have told the hat promptly to put her in Slytherin.
Loyalty? Yes. But Hufflepuffs also pride themselves on tolerance, friendship, kindness and fairness. None of which are qualities Bellatrix has.
Headcanonsandmore 