Game etiquette

So what’s the customarily accepted code of behaviour here.

I attacked country A , took out 50 provinces and all of his troops. However, he had moved his capital as far from my attack as possible, next to Cuntry B.

As I was about to take the last province, his capital Cuntry B snipes the province and takes it.

My Q. Should I be pissed I’m thinking he doesn’t deserve to stay in the game so he’s about to be taken out of the game. Overreaction??