If I say that Harry is not going to kill people if he listens to Hufflepuff but might kill if he listens to Slytherin, would that be a strange claim for you? If I say people who always listen to Hufflepuff don't go on killing sprees would that seem strange to you?

No and yes respectively.

Hufflepuff isn't a natural category, Harry!Hufflepuff is an abstraction based on Harry filtering his personality through certain criteria and impulses, such as what he conceives of as loyalty and compassion. Do I think that Harry, reasoning through his conception of loyalty and compassion, would go on a killing spree? Unlikely. Do I think that there are people who, reasoning through their conceptions of loyalty and compassion, would go on killing sprees? Absolutely.

A neurological fact that may be of some relevance here. Oxytocin, the chemical associated with triggering feelings of love and affection, has also been found to trigger increases in xenophobia and ingroup/outgroup bias.

Feelings of love and loyalty are not anathema to hate and violence. Rather, they often go hand in hand; the same feelings that unite you with a group can also be those which make you feel you're united against something else.

Lastly it's useful to keep in mind what extraordinary claim needing extraordinary evidence can lead to. If you take it too far it shuts down people from saying what they honestly believe and instead let's them argue beliefs that they don't fully stand behind.

How so? I don't take any issue with your stating your beliefs and arguing in their favor. As is, I think that they're misguided, but that's because I think the weight of evidence is not in their favor. If you convinced me it were, I would change my mind. I think it would be far more useful for you to defend your belief with the best evidence you think favors it than to simply assert your belief.