Oh boy I actually have an entire compilation in my notes app

"We can affirm that there is not a single article of the Convention of The Hague or principle of international law that the Bulgarians did not violate." — Report of the Inter-Allied Commission in Eastern Macedonia"

"Bulgarian policy in Macedonia, and to some degree in occupied Serbia, was motivated by what historian Alan Kramer has termed a ‘dynamic of destruction’ a desire not just to defeat the enemy militarily, but also to erase all traces of its culture and destroy any evidence that it had ever been there at all.[2] In order to create pure Bulgarian territories, the Bulgarian military government started implementing in eastern Serbia, Macedonia, and parts of Kosovo a political system of systematic denationalization, Bulgarisation, and economical exploitation"

"In the eastern parts of the region, where a considerable part of Macedonian Slavs had pro-Bulgarian sentiments or felt themselves to be Bulgarians, that population welcomed the Bulgarian army as liberators. For the rest of the population and in particular for the Macedonian Slavs who identified as Serbs (or those who felt neither Serb nor Bulgarian), the brutality of the Bulgarian army, the irregulars Komitadji and the later civil administration had all the features of ethnic cleansing."

"Bulgarian Tsar Ferdinand declared on the eve of war: "the purpose of my life is the destruction of Serbia"

"The Documents relatifs aux violations des Conventions de La Haye et du Droit international, commis de 1915–1918 par les Bulgares en Serbie occupée, a report covering alleged atrocities committed in Serbia, published after the war, stated that ‘anyone unwilling to submit him or herself to the occupiers and become Bulgarian was tortured, raped, interned, and killed in particularly gruesome manners, some of which recorded photographically'. Bulgarian units that occupied Serbian territories showed extreme brutality, systematically expelling the non-Bulgarian population in the regions they occupied, they arrested the population and set the rebel villages on fire."

"Bulgarian policy was so rigid that it even provoked mutinies among its own soldiers."

"In February 1917, a spontaneous Serbian uprising broke out, in the Bulgarian occupied territories of southern and eastern Serbia. (...) On 10 March 1917 Protogerov issued an ultimatum to the chetniks to surrender within five days or face execution. They did not surrender, so Protogerov and his army attacked the civilian population and their villages.[38] About 20,000 Serbs were killed in fighting, executions or reprisals.[37] In the town of Surdulica alone about 2,500 Serbian men were executed, thousands of women and children were interned and others sent to prison. Thirty-six villages near Leskovac were completely depopulated."

"In ex-Serb Macedonia, for the first time in history, gas chambers were used for the purpose of mass executions, exhaust pipes of trucks were attached to sealed sheds by Bulgarian soldiers where they herded the Serbs whom they wished to eliminate."