I had an incident just like that, except it was with an RTS.
I was over at a cousins house with 3 of my cousins. Lets call them Cousin 1, 2, and 3.
Cousin 1 decides to have a LAN party of Age of Empires and we all agree. Cousin 1 is whooping all of our asses repeatedly for several games, and Cousin 2 and 3 are attempting to ally with him every game.
In one of these games Cousin 3 has his whole base not too far away from mine with no water separating us. He has built walls but left a section open for his civilians to get through to for supplies even though there is a gate feature. For some reason he thinks we are allies, even though we are neutral with each other. So I decide I want his land and take a mass army to his base.
Did I mention all 3 of my cousins are in the same room while I am downstairs? We are all using headphones to communicate though.
So while I raid his base and am in the process of completely raping him he starts screaming about how we were supposed to be allies. He then gets up and runs downstairs and bursts into the room I am in.
He practically starts choking me in my chair and reaches for the power button on my computer, what ensues is basically a power struggle with his finger reaching for the button. He wins and I regret not being able to complete destroying his base...
Needless to say we weren't allowed to play LAN again for a while >.>