There's a fine line between being socially awkward, even being brusque to cover for that, and being a dick for the sake of being a dick. It's like the MC long ago said, "Hold my glasses," and took a huge running leap over that line. He goes well beyond even what's acceptable for a power fantasy, and does so with virtually every spoken line.
why the fuck would you do it yourselves now?
The important difference is which side the ruler and her court are when the bridge collapses. If the humans broke the bridge during the "peace" talks, then they would have been stranded in human territory. With them safely back home, breaking the bridge makes it harder for the humans to invade.