I have mentioned a number of times that I believe to understand OD&D and B/X it is important to remember that they are outgrowths of wargaming and carry a lot of assumptions from wargaming.
One of these assumptions from wargaming was that you knew who the enemies were. In a historical wargame they wear different uniforms and are controlled by another player; in OD&D and B/X they were labelled "Chaotic" which, by default, made it okay to kill them.
The other aspect of alignment is a mechanical one. Spells such as Protection from Evil need to know who the bad guys are.
As roleplaying matured, as a genre, the labels got stronger ("Evil") and the characters were clearly portrayed as "the good guys". The paradigm shifted towards telling a story, and with that came the assumptions appropriate to a story - i.e. that the good guys would always grow in power and triumph over adversity, and the death of a good guy would be a rare and tragic event.