Politeness/civility/conversational kindness won't always save you, but here are some points that make it very much worth doing:
If you try kindness first, you always have room to change tack without it rebounding on you too badly and fouling up your future efforts.
If you act like a shitty little tool, you are going to have a hell of a time walking that back and have probably screwed the pooch completely if it is a low stakes thing, like the example above, where there is no need to form a basis for further interactions. They're going to write you off, and will frequently inconvenience you if they can, no matter how mildly.
Being chill and being shitty are both satisfying in different ways, but only one of them actually advances your mood in a way that is going to feel more than momentarily vindicated.
Being shitty, punishing people, or being difficult or assholish from the jump, is very satisfying in the moment. But it doesn't really work out long term. You just wind up in a shittiness loop, being a dick to the world and getting dick back, and thus perpetually feeling like shit re: your position in the world. It is so bad for you to just...hate people. That's a thing that by the time you will openly say it has already turned against you and is eating away at your stability and ability to love and help yourself.
Being decent can end an annoying interaction with less conflict, which means less stress hormones or whatever, and your body literally will feel worse and form an instinctive stress response to people. Shit loop.
When it doesn't really reduce the conflict, you still have less stress hormones, AND anyone who sees you will be way more likely to think the other person is an asshole, meaning their conses have gotten way more likely to quence because they just hung their whole ass out.
Being decent FUCKS with people. Just. Fucks with 'em. They don't know what to do with it, they may back off, reality check themselves, bluescreen, or suddenly reveal a completely different emotion that was at the root of what you saw and did not like. They can't walk away and say honestly to themselves that you were rude, that the way you treated them justified the way they treated you. They may TRY to tell themselves that, but the majority still know it is a lie.
Being shitty doesn't make people change. LOVE isn't always enough to make someone change. You absolutely cannot hate people into changing. Being shitty does make them more likely to double down.
You have planted the seeds in them of realizing they are acting badly. Not all will fully realize this. The ones that do learn more from kindness than anger/cruelty.
Being cool about shit is better for you. You can give zero shits about them and still benefit from being more easygoing and less hate filled. It doesn't have to be about morality or them being people who deserve respect. It can be about not being stressed all the time because you believe you are surrounded at all times by terrible people.
I prize efficiency in interactions where there is friction. Being easygoing is extremely socially efficient.
They may behave completely differently the next time. We got dogshit service at a hotel restaurant once, but it was one we loved, and servers don't get paid enough so we still tipped generously because even an asshole might have a dependent who relies on that tip. ALWAYS TIP I know you hate it, choke it back, it isnt about teaching them a lesson, this is solidarity with the working class. Unless they are personally fucking horrible to you, like bigoted or whatnot, tip them. The next day when we went back, the same person was SO GOOD. They knew they had not done their best, I think, and proceeded to do much better. And they probably went on to have a better day and be better to people.
You do not know what the hell is going on behind the scenes. Had a nurse be kind of bitchy to me once, but I trusted the clinic and other staff, and knew they would not tolerate an asshole among them. I was doing okay that day so I was polite and respectful and as efficient as possible, and let her WEIRD behavior be her problem. Next time I saw her she was really sweet and has been ever since. There was a shadow there. What the fuck had happened that day she was unpleasant? If it had been a pattern, that is different, sometimes people are shitty. But most people are not. They're just human.
You can't get to know what people are like when they are not being assholes if you are being an asshole.
Being an asshole by default and being shitty in interactions makes you look really...I don't know the word. Weird, bad? "What's HIS problem?" "What's up THEIR ass?" "Who pissed in HER Cheerios?" It others you. It makes you look at fault. Unreliable. Unstable. Someone who can be safely dismissed.
Being nice sets a good or at least neutral baseline. Deviating from that has more impact. Being sharper when you are usually chill then becomes a means of emphasis, a signal that something is truly wrong, and not just your nature.
Seeing the better side of people is great.
Nobody anywhere ever has been perfect. I do plenty of the shit above when I am under pressure. But leaning into forbearance has made me much happier than being angry ever, ever did .
You may not make progress quickly and may not be good at it because you have not learned to be. Be open more often, though, when you can. You will be surprised to find out how many people respond very well.
People absolutely will not remember exactly what you said or did. They may not even remember your FACE. But they will not forget how you made them feel.
I am firmly of the opinion that we have to love other people at least a little (but often) or we lose ourselves to hopelessness and hate, and apathy, and those are miserable to live with inside you all the time. And they make the world so much worse.
People are not innately bad. You're just emotionally worn the fuck out and don't communicate with others often and openly enough to help overcome that.
Ugh. I am so passionate about my love for humanity. I'm sorry if this didn't make sense or words or sentences were weird. I am VERY sleepy. But had to say that hate didn't come free with your humanity. Your humanity was stripped from you by a hard world and hard life, you hurt because you feel strongly, you aren't wrong to be defensive and angry. But you deserve better. And in the end? In the end, you absolutely must participate in repairing yourself. The world can't do that for you. Not won't. It literally cannot. And you deserve better than a bitter life spent around living beings you hate without knowing.
Please please try to want better for yourself. It isn't about becoming a more moral person, it's about having a place in yourself for the good things in life, so you do not miss things that might bring you joy.