Cruelty towards those who are relatively weaker or indeed defenceless - children, the elderly, partners, animals.
Abusing service / customer-facing staff fits in to this as well and is at once particularly revealing and particularly damning. Next time you’re out and about with friends or a love interest watch how they treat (or talk about) e.g. the person at a ticket booth or the person waiting on tables - if they’re nasty to them (or about them), imagine what they might be like behind closed doors (and how they might treat you one day).
Abusing service / customer-facing staff fits in to this as well and is at once particularly revealing and particularly damning.
Whenever a customer is rude to me I just remember that they're probably compensating for how terrible their own lives are. If it was actually an issue on my part then one of my coworkers would've told me by now. Makes it way easier to move on with my day.