I am curious about the idea of calling sila (all precepts and vows) as ethics. When did this translation appeared? Or who interpreted sila as ethics or something similar to ethics? This is not to say that it is incorrect or comprehensive, but ethics as a western category of thought has been used as an auxiliary concept to understand sila. Which scholar/s did this?
From the very little I know of history it was the Pali Text Society who made the earliest surviving translations into English and the earliest dictionary.
A copy of the PTS dictionary is online here -- https://dsal.uchicago.edu/dictionaries/pali/ -- though this seems to be the latest (2001) not the earliest (1874) version.
The dictionary's entry for sīla is here -- https://dsal.uchicago.edu/cgi-bin/app/pali_query.py?page=713 -- "ethics" is a part of the definition but only a part.
I think that's normal:
That any word has no exact translation -- which is why people try to learn to understand the original word or language
That a translator tries the convey the sense of it by giving several 'related' English words.
Often the several words (in a dictionary definition) give a better or closer explanation than any one single-word translation, which is part of why a dictionary can be interesting. When translating a text the translator usually needs to choose single-word translation to make the result read like English -- so different translators might translate a text differently, etc.
I don't know "the western category of thought": I never studied it academically, so I can't comment on that. If instead you translated sīla as "virtue" or "moral conduct", I'd say those means approximately the same thing, to me they're near-synonyms.
The word exists in Sanskrit -- https://www.wisdomlib.org/definition/shila -- I presume people began to translate that earlier than Pali, e.g. Thomas William Rhys Davids studied Sanskrit at a university in Germany, as a teenager.
Bit of a pedantic question I guess but morality, moral discipline, ethics, ethical conduct, these are all synonymous. While in actual definition ethics is different from morality but the idea of the moral discipline, like not killing is a result on understanding ethics, and having ethics.
If one only does not kill simply because "Buddha said so" or "I practice the 5 precepts" or "I made a vow not to kill" that is just practising morality. Being ethical, i.e having ethics that would not allow you to kill another being is a separate phenomena, and I would guess is the essential meaning of the moral discipline. So the word ethics is better suited as a whole.
Still this is a triviality of semantics, understanding the concept is the more important thing.