How people treat you proves THIS
If they treat others with kindness and respect, chances are they respect and are kind and compassionate with themselves as well.
If, however, they treat others terribly, deep down they're likely unhappy with themselves and, more generally, with the life they lead overall. Narcissists tend to fall under this category because even though they may project an air of self-confidence, they're really trying to overcompensate for self-doubt and low self-esteem.
It's been said that misery loves company. If people are feeling miserable, they often look for ways to upset others so those negative vibes can rub off on them. Maybe they tell you that you look fat in that dress, they leave you hanging with no explanation after you both agreed to meet up for drinks, or they reel off a list of your past mistakes -- all to get under your skin.
Sure, you might simply catch someone at the wrong moment -- maybe after their girlfriend broke up with them or they learned they were being laid off -- and they end up taking it out on you. Hopefully, whenever it is they cool off, they come and apologize to you.
But there are those who are perpetually negative and spiteful. They seem like they're always trying to concoct a plan to undermine or embarrass you. Below the surface, they might very feel like others have undermined or embarrassed them, so this is their way of evening the score -- even if the target had nothing to do with it.
How a person treats you is a good measure of their character. If they don't exhibit warmth or compassion toward anyone, do you really think they love themselves?
My guess would be no. If you don't love yourself, you can't love other people -- it's as simple as that.
And if someone treats you like garbage, don't stoop to their level. Be the better person by walking away and surrounding yourself with positive individuals who bring you joy rather than heartache.