I personally know several people from Van (one of them was a revered teacher of mine) whose families were victims of Armenian atrocities. I have also talked to several people from Erzurum and Erzincan who have first hand information about these events from their grandparents, relatives, and neighbors. Of course, personal knowledge and conviction such as mine do not constitute "proof" for others; so we quote expert historians and verifiable documents. The following was published by Prof. McCarthy.
[Captain Emory Niles and Mr. Arthur Sutherland were Americans ordered by the United States Government (in 1919) to investigate the situation in eastern Anatolia. Their report was to be used as the basis for granting relief aid to the Armenians by the American Committee for Near East Relief. The following are two excerpts from their report.]
"In this entire region [from Bitlis through Van to Bayazit] we were informed that the damage and destruction had been done by the Armenians, who, after the Russians retired, remained in occupation of the country and who, when the Turkish army advanced, destroyed everything belonging to the Musulmans. Moreover, the Armenians are accused of having committed murder, rape, arson and horrible atrocities of every description upon the Musulman population. At first we were most incredulous of these stories, but we finally came to believe them, since the testimony was absolutely unanimous and was corroborated by material evidence. For instance, the only quarters left at all intact in the cities of Bitlis and Van are the Armenian quarters, as was evidenced by churches and inscriptions on the houses, while the Musulman quarters were completely destroyed. Villages said to have been Armenian were still standing whereas Musulman villages were completely destroyed."
"In this region [Bayazit-Erzurum] the racial situation is intensely aggravated by the proximity to the frontier of Armenia, from which refugees are coming with stories of massacres, cruelty and atrocities carried on by the Armenian Government, Army and people against the Musulman population. .. Here also the Armenians before retiring ruined villages, carried out massacres, and perpetrated every kind of atrocity upon the Musulman population and the doings of the Armenians just over the frontier keep alive and active the hatred of the Armenians, a hatred that seems to be at least smoldering in the region of Van. That there are disorders and crimes in Armenia is confirmed by refugees from Armenia in all parts of the region and by a British officer at Erzerum."
Note: The entire report can be found (as a pdf file) in the web.