As far as ethics is concerned, it is a bit difficult to determine clearly if Dr. Yu Tsun was ethically culpable. Ethics is a branch of philosophy which seeks to address questions about morality. At the same time morality In its first, descriptive usage, means a code of conduct which is held to be authoritative in matters of right and wrong. Morals are created by and defined by society, religion, philosophy or individual conscience. In other words, the morals by which the actions of Dr Yu Tsun are to be tested depend on, profoundly on the side of the contender (by this I mean which side of world War I are you on).
In the case of being on the side of the Allies, the death of Stephen Albert is a horrendous crime and thus it is easily concluded that the actions of Dr. Yu Tsun are ethically incorrect. Nevertheless, if we observe the facts from the German perspective, we can think that the idea of using Stephen Albert as a medium to communicate valuable information then, by implying that in war, any action that benefits a combatants cause is justified. Moreover, had Dr. Yu Tsun not been arrested, he would probably been decorated for his actions.
On the other hand, philosophically, as Socrates describes to Crito in prison awaiting execution, harming someone with premeditation is always considered evil and evil acts are non righteous which makes us conclude that philosophically, killing someone under any circumstances is always wrong.
Considered from the most obvious ethical perspective, Dr. Tsun is definitely ethically responsible for Stephen Albert’s death. He looks him up in a phone directory, finds him, and kills him as he had been planning. The question is more complex than this, however, because Borges has set up a text world that casts great doubt on the nature of human existence and agency. If we are led by chance and coincidence, and if we are present simultaneously in multiple (or infinite) worlds, where all of our possible decisions are played out, do we really have any choice but to do what a particular “universe” demands of us? The basis for ethics is challenged, which is at the heart of this question.