How to think assertively on the spot, or some stuff like that

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I just went to the supermarket and there was a mother in front of me with her spawn, I was waiting for my turn to pay and the spawn makes a gesture to put his shit on the counter, thus skipping queue ahead of me, but has a moment of afterthought when he turns and sees me. We make eye contact and after a second I'm like sure, go ahead. Kid keeps chewing his bubblegum in silence while his mother doesn't even give me a look, let alone say "thanks", and start chatting with the cashier like I don't even exist. Of course I wasn't expecting any thanks 'cause I knew they were subhuman scum but why was I such a faggot and let them skip over me? They weren't even *trying* to skip, I actively told them to because I caught on that hint and acted upon reflex.

And here we get to my issue. How do I stop being like this?

I have NO problem whatsoever being assertive with people I know. I'm a bit soft with SOs but when it matters I have no trouble at all saying no. If I'm given time to think about something, this issue completely vanishes. But with strangers, especially when it's an *act* instead of a verbal exchange, it's like I'm forced to act proper and give my seat, or let them skip queue, and so on. Most of the time this pleases me because I like being kind with others, but there are times where I get walked on, or I service shitty people who don't deserve it (like the example above). Is there a way to train myself to act kindly as a choice so I can stop myself from doing it in those 1% of cases?