It is a historical fact that the European powers engaged in open drug trafficking. It is also a historical fact that, subsequent to the Boxer Rebellion, European powers imposed treaties to "assert a right to promote Christianity" and "[accord] special privileges, extraterritorial rights and immunities from Chinese law" (such as anti-Opium laws). On these points, both Chinese and Western historians agree.
However, there seems to be a gap in the modern and historical definition for the killings of foreign missionaries, diplomats, and businessmen preceding and during the Boxer Rebellion between Chinese and Western sources.
All the popular Western sources I could find (Wikipedia, Reddit, BBC documentaries, Youtube commentaries, etc.) refer to the events as "murders", all Chinese sources refer to these events as "justified killings" or "self defense against foreign invaders/bullies." The two definitions seem to be polar opposites. Which is it?
Or is there some kind of middle ground definition in academia?
It's ultimately a subjective judgment, like the "One man's terrorist is another's freedom fighter" of later years. The use of "murder" was of course common in western circles at the time but later reinforced by the language of the 1901 Protocol in which the Imperial authorities accepted that crimes had been committed (but whose English text curiously labels von Ketteler's killing as both "assassination" and "murder", and Sugiyama's as assassination only). Today such deliberate killings of non-combatants would generally be considered illegitimate, but it remained a common tactic of 20th-century armed national liberation movements, often officially forgiven at the end of the conflict. I think historians would tend to avoid any suggestion of justification of individual killings of civilians, though the uprising might itself be defended in all of its chaos: I'd avoid "murder" too in such contexts as implying an act directed specifically against a person as an individual rather than as a symbol, an office-holder or a representative of a group, though for instance the Cambridge history of China speaks of von Kettler's end as a "death", a "killing" and a "murder", the last in connection with the powers' acceptance of a German commander.