Your reputation as a magnanimous and kind person could make a new person on the scene think that they can take advantage of you: ha! The joke will be on them when you prove, in dramatic fashion, that's not the case today. Don't be afraid of coming off as rude to people who underestimate you. They did you the disservice of underestimating you, so there is no need to bend over backwards to be polite. This is admittedly a rocky start for the two of you, but things will steadily improve.