“A person who is nice to you, but is not nice to the waiter, is not a nice person.”
People who are mean to those who serve, those who have less, those who they do not feel can help them, or directly benefit them are not nice people.
They are users.
They are self-centered.
Possibly narcissistic, and possibly socio/psychopathic, lacking empath and likely compassion.
(Which, in and of itself is not bad, but without compassion, it can be a nasty thing.)
“A person who has only mean things to say about their exes will probably eventually only have mean things to say about you.”
When a person has been the victim in every relationship they have, and chooses to focus on only the negatives with their exes, or place all the blame on others may not be that way to you right now—they weren’t that way with their exes to begin with, either, I’m guessing.
(Sort of hard to get into a relationship by being overtly asshole-ish, although some manage it.)
However, if you have a falling out, know that’s how they will probably talk about you.
And that’s probably how they talk about you in their head, too. When they’re not getting what they want.
Is that the energy you want in your life?