This is just a prefatory note to something lengthier. You have been warned.
The 'Bulgarian horrors', and Gladtone's response, have been cited a number of times in prehistories of 'humanitarian intervention'. For example, Martha Finnemore cited it in her 1996 essay, 'Constructing norms of humanitarian intervention', and Gary Bass cites it again in Freedom's Battle: The Origins of Humanitarian Intervention. I have alighted enough times on the simple wierdness of advocates of humanitarian intervention seeking validation for such a programme in 'Old Europe'. As the two examples mentioned indicate, this trend is not restricted to the vulgarising, coarsening rhetoric of the belligerati. It is also evident in serious scholarship, such as in the work of Neta Crawford (see some astute criticisms in Patricia Owens' review [pdf]).
But, specifically, what is it about the British Empire and the 'Eastern Question' that seems so susceptible to such a reading? After all, there is no doubt that the institution of race was a crucial normative factor justifying calls for intervention, whether in the lurid pamphlets of Gladstone or in the letters of Bishop Strossmayer of Zagreb (whose reading of the Koran in his is October 1876 correspondence is quite similar to that of Sam Harris, by the way). Moreover, it is precisely through this institution that the impassioned moralism, the 'humanitarianism' itself, was convoked and expressed. Gladstone's "pilgrimage of passion", as his detractors called it, was itself both a phenomenal display of electrifying wrath-of-god popular agitation (a mode of communication which Blair sampled and looped, causing some liberal and neoconservative commentators to lose both mind and underclothing) and a vulgar racist crusade against Islam. (This liberal imperialist allowed that the Mahometans may be manageable when a subordinate minority, as in British-ruled India, but in Turkistan the deficiencies of Islam became all to evident). Both Finnemore and Bass are aware of this, and duly embarrassed by it. After all, if Finnemore was right and a new humanitarian norm was being defined in this era (though she hastens to add that this was evident more in justification than in policy), this would confirm that this norm was being constructed as an aspect of that ascriptive hierarchy known as 'race' (and the contiguous hierarchy known as empire). It would also tend to support the point made by Marc Trachtenberg that "To be a target of intervention—indeed, even of humanitarian intervention—was to be stigmatized as of inferior status". And that, of course, undermines the assiduously constructed narrative according to which humanitarianism in the context of imperial foreign policy represented the successful intrusion of egalitarianism into foreign affairs.
Yet, the temptation to scour the annals of Old Europe, particularly those instances in which there is a putative clash with Islam (Greece in the 1820s and Lebanon in the 1860s are the other two key examples that tend to be cited), persists - and it has to be read symptomatically.