He should just have mated you. And I agree that if you want to resign you should normally do it when it is your turn.
I actually find it rude to resign one move from mate. I like to mate! So there is no commen idea of when to resign or not. So what you find right!
In a recent game (against someone a fair bit stronger than me), towards the end of the middle game / beginning of the endgame, I made a blunder and gave my opponent a mate-in-1, which almost immediately after me having made the move became obvious.
My opponent, instead of making that move and ending the game, waited for a bit and then wrote in the chat that I should resign, because it's mate-in-1. Because I refused to resign, waiting him to make the final move, he considered me extremely rude and impolite, essentially a jerk (which he made amply clear).
I find the notion of not resigning on your opponent's turn being considered rude and impolite to be extraordinarily baffling and incomprehensible.
For quite a long time, since my days of playing Go, and even before, I have learned the principle that it's the exact opposite: Doing a move and then resigning on your opponent's turn is what's impolite and rude, not the other way around.
It is a common (but not mandatory) practice (or at least was at some point, might still be) in the grandmaster-level Go world (and probably all the way down to students), especially in Japan, that if you are in a clearly lost position and intending to resign, you make a last move, wait for your opponent to correctly respond to it, and then politely resign. Making a move and then immediately resigning on your opponent's turn would be seen as quite unusual, inappropriate, and probably rude. It would be something very strange to do.
I think it applies to all these turn-based tabletop games. After all, quite often your opponent may be thinking about his move, making sure that it actually works and that he doesn't blunder, then when he's sure that it's the correct move he makes it, and then you politely resign, as if saying "yes, that was indeed correct, you played well, I have been bested, I concede".
However, resigning on your opponent's turn I would consider rude and impolite. It's potentially interrupting your opponent while he's thinking, and it's like wordlessly telling your opponent "you know what? I'm actually not even interested in what you are going to do, I'm just ending it now. Whatever you were pondering, it doesn't matter, and I'm not interested in it." It's a bit like interrupting someone in the middle of him explaining something and rudely saying "just shut up, I'm not interested, I'm going away now."
Thus, I hold as my principle to never resign on my opponent's turn. I always wait for them to make a move before resigning, no matter how clearly I have lost. Even if it's a glaringly obvious mate-in-1 that even a 300-rated player can easily see. I'd rather give my opponent the satisfaction of executing the checkmate than interrupting them as if their move didn't matter.
Thus, I find the behavior of that other player baffling. He seriously considered me rude and impolite for not resigning on his turn. For me giving him the opportunity to deliver the checkmate at his own leisure, me waiting patiently for it, without interrupting him.
Why would anybody think that it's impolite for your opponent not to resign on your own turn, no matter what the board situation may be?