In 1839, the UK & Belgium signed the "Treaty of London" one of the points discussed in this treaty is that the UK would be there to defend Belgium in the event of war. In 1914/15 Germany instigated the "Schlieffen Plan" whereby it would go through Belgium to get to France & attack it. Going through Belgium put the "scrap of paper" into place thus entering Britain into WW1.
However during the 75 years between the treaty being signed & it been acted upon many developments & opinions had taken hold in Europe; Queen Victoria had been & gone from the throne, the first stage of the Industrial Revolution (1760's - 1840's) was shifting into it's second stage (1870's -1910's) & the "scramble for africa" was in full swing.
Belgium's colonial atrocities in the Congo Free state are well-known under the leadership of Leopold II & the UK has a fair few atrocities of it's own (the Boer war, Boxer Rebellion, 1857 Indian mutiny etc) did these atrocities ever compromise the 1839 agreement?
How did each country's society react to the other's atrocities?
First, we should understand what the 1839 treaty said [e: and why it was a fairly passive thing]. Scott Manning has the full English text, which unlike some treaties such as Wuchale between Italy and Ethiopia in 1889, conformed pretty well to its non-English counterparts. The treaty itself is short (many powers with Belgium), but the multilateral annex agreement is longer. Much of it is dull mechanics, dealing with dividing or respecting previously shared rights with the Netherlands and various elements of statecraft relative to Britain and others, with Article VII of the annex (between Great Britain, Austria, France, Prussia, Russia, and The Netherlands, not specifically Britain and Belgium) being the most important for 1914--the guarantee of perpetual neutrality, supposedly incumbent on all signatories. There's nothing in there about behavior within one's state, or towards one's colonies; the British certainly wouldn't have accepted such limitations because they would seem insulting, and Belgium didn't need to--until 1908 they technically didn't have any colonies of note.
See, the difference here is that the Belgians didn't technically commit any colonial atrocities: Leopold's International African Association, under King Leopold II and only in a personal union with Belgium, was the responsible organization in terms of the Congo Free State. British individuals and statesmen (Casement) were involved in the exposing of the Congo Free State, and when Belgians became aware during the campaign, they demanded an end to it as well. The Belgian government took over in 1908 as a "responsible" move, and indeed the raw savagery ended though the basic exploitation really did not. (The history of the Union Muniere and the entire Katanga mineral belt--imagine diacriticals there--is quite enlightening on the point, but I'll have to see if there's a good one in English.)
If one looks at the power differentials, it also explains why Belgians wouldn't repudiate such a "recognition" treaty over the Rebellion/Revolution/Mutiny in India (which, again, was under a non-state entity: the East India Company), the Boxer Rebellion (which most of the signatories took part in suppressing), or the South African War and its camps/scorched-earth policy. The British, on the other hand, would be hard pressed to withdraw from a multilateral treaty recognizing Belgium's autonomy over something done by the monarch separately under another legal umbrella.
The case of the SA War was arguably the most challenging because it tapped a distinctly different nerve in the Low Countries, but there is no evidence that in Belgium or The Netherlands (per Martin Bossenbroek) there was any official sanction against Britain for its actions in what was, to them, a recognized British sphere of influence and suzerainty under the Berlin agreement. I've never seen any discussion from the Belgian Consuls in SA of the treaty, but then they never even toyed with intervention in South Africa, though there are notes of concern and seeking information about the war's proceedings as well as safeguarding Belgian nationals. Public sympathy tended to be with the Boers, and Dr. WJ Leyds (head of the South African Republic's foreign services in Europe) helped to make sure it was heard.* However, state interests were not moved. I have to get back to my library before I can tell you how many Belgians actually fought on the Boer side, whether or not in the "foreign legion" they [edit: the Boers] set up.
So whether or not popular or official disagreement existed, it did not rise to the level of repudiating a treaty as (at the time) nondescript as the Treaty of London. That was, after all, just the legal recognition of the state's rights internationally, not a true alliance or privileged status to be revoked. That's part of why the Germans were somewhat surprised that this specific "scrap of paper" was invoked to bring Britain into the war in 1914.
[* edit 2: Leyds also made sure the record of that effort, and the buitenlandse gezantschap itself, was well preserved; it's a massive archival collection within the Transvaal Archives in Pretoria all on its own (class LEY).]