Yeah. I think its important to remember that this "bouncer" brought a knife to a fist fight. Regardless of whether he deserved to die, and all this discussion of evil or not, he escalated the scenario. He turned the scene deadly. He did. At least how I read the OP. And I admit I may likewise be bringing my own bias into it. But that's how it seems to me.
Agreed, but we aren't judging the alignment or the wisdom of the bouncer. But as far as that goes, the bouncer at least has the excuse that it was his duty to protect the bar and its patrons. We don't really know what provoked the fight, nor do we know the bouncers full motivation, but the PC's were clearly armed and armored when the fight broke out.
As inexplicable as the bouncer's conduct may be, the battlemaster's conduct is even more inexplicable. The battlemaster was apparently content to meet deadly force with subdual force the entire time he was threatened by the bouncer, but once the bouncer had yielded and was no longer a threat, then and only then did the battlemaster resort to lethal force. It's a lot easier for me at least to by sympathetic to the bouncer than the battlemaster. What happened here is
at best, equivalent to a cop who gets attacked by a guy with a gun, decides to out of apparent hubris disarm his attacker with his bare hands, and then only after the attacker is disarmed, has his hands up, and is begging for mercy, does the cop pull out his sidearm and pop the attacker in the face because "he tied to kill me". Even more so than the cold blooded nature of the murder, the reasoning to me is telling.
And I say, "at best", because it's not clear to me that the Battlemaster is socially, legally, or morally equivalent to a magistrate of the peace.
Generally speaking, in the real world if you find yourself caught in a bar fight, if anyone dies you'll be charged with murder. Doesn't matter who threw the first punch. Doesn't matter what the other guy does. In most cases if you are in a public place (ei, not in your home), you have a duty to retreat and anything you do to escalate the situation will make you culpable in the outcome. Self-defense will be in most cases not considered a valid defense. And self-defense is certainly not a valid defense after the imminent threat to your person has ended.
In a culture divided into nobles and commoners, the aristocratic status of the Battlemaster might protect him depending on the specific laws. But killing someone in cold blood like that is no less murder. The range of alignment options we have for the Battlemaster are "anything but good", but the action itself is evil.