That’s a fair point. That said, I expect that Cap was similarly gleeful during WWII when the Nazi army was pushed back and territory was retaken–and that would have involved plenty of casualties. It’s the cost of doing business. Given the number of worlds that have already fallen, and the number of beings killed up to this point during the battle with the Builders, I can forgive Cap for showing some enthusiasm at the point where things begin to turn in the heroes’ favor.