Wow, that's an equivalent to, "I'm going to go tell my big brother on you, then you'll be sorry."
I can already see it now. He gets to the imperial castle to snitch on the protagonist, probably with a tearful expression or a sad face at the very least. Telling the emperor that his daughter was taken, or killed, and that it was MC's fault.
Only for the MC to appear with the princess and be all like, "you what, mate?" And explain that not only was it his betrayal that got the princess kidnapped, but he also fled a battle that they ultimately won.
Can you see the look of anger on his face? I can, and it is glorious.