Did the Turks have some historical dislike for Armenians in 1915, or was it just the circumstances of WWI?
The Turks had launched several persecution campaigns against their Armenian subjects in the 19th century. This usually happened in tandem with and under cover of their wars with Russia. Armenia had been an independent kingom before the Ottomans, and the Turks always feared a resurgence of nationalism. Once the Greeks achieved their independence in the 1820’s, this became an even more palapable concern (especially to a homocidaly paranoid ruler like Abdul Hamid). Also, persecutions gave the Sultan an outlet for his rootles unemployed - instead of potential riots in Istanbul or revolts in Kurdistan, they could be sent to unruly provinces with open invitations to loot and rape.