it does seem that our natural tendancy is to try to placate people and make sure that at the end of the day this person still likes us.
Yes, true. Or that everything will still be “nice” and warm and fuzzy, and everybody can all smile and feel okay.
"So, please help me, ehell, and let me know a polite way to make this person stop the bad behavior because I can’t do it by myself and I don’t want the person to be mad at me."
It doesn't help that, for many of us (especially with family), the ire is going to fall on the person who's seen as causing the disruption, often for "not being nice." How many times has my father taken me aside quietly and said if I were just nicer and politer to my mother she wouldn't get upset and throw tantrums so often? Or for us young ladies, if you had just been a bit nicer and flirted a little instead of saying no that guy wouldn't have called you a female dog. There's often a lot of external pressure to blame the person who triggered the outburst for not being nice enough, because we're the ones people can control!
Sadly, this is true. When I first started standing up to my father, he'd tantrum. And everyone else in the family would turn on me, "Look what you did! You got Dad all upset! You've ruined Christmas/the birthday party/dinner."
But by that point, I had had it with always being blamed for everything that went wrong. "No," I'd reply, "Dad is making himself upset." No one knew how to handle me, once I simply and flatly refused to take the blame for Dad's temper tantrums. I remember telling a few people, "You know, Dad's reaction is not normal. Normal people do not turn red in the face and scream for 15 minutes because their adult child can't leave work in the middle of the day to drive them to the store with no prior notice. Normal people do not sulk for two days because someone didn't like the way they cooked the green beans last Sunday. I'm not being mean to Dad. But I am being realistic here. He's the one with the problem, not me. And I can't fix it for him. No one can but Dad himself."
If you've grown up in a house where everyone walks on tip-toes around one person, where the family watchword is "Don't get Dad/Grandma/Aunt Susie upset. You know what will happen," it's very, very difficult to break training and just let that person get upset. It's very hard the first couple of times, and the blame from the rest of the family doesn't help. But it goes a long way to developing a nice, sturdy, shiny spine.