Rainbow Road:

I assume you're talking about your recent game with vacgo.

There could be a couple different reasons why.

One reason may be that vacgo is a relatively high-rated player and may be under the impression that the game score affects the ratings of the two players. On most go sites (e.g. cGoban has a mostly real-time system), final score greatly affects the two players ratings, not just who won and lost. I'm not sure whether or not BrainKing's rating system uses the score, but my guess would be that it doesn't.

Another possible reason is that he wants you, and anyone else who might look at the game, to understand how to score it properly. Playing a game by the wrong rules can get make for some bad play or frustration. I'm not saying you don't know how to score, but maybe he was under the impression that you didn't from not marking those stones as dead. I've seen some people playing on this site with a very bad understanding of the rules, which are a bit unorthodox compared to games like chess or reversi, and he may deal with it regularly.

I doubt it was an ego thing for him, but I've certainly seen worse things from people.