Well, there are no real good reasons for genocide. But, as for the reasons used by the Turks to justify it...
-The Armenians were Christians, while the Ottoman Turks were Muslims. The Turks had used this difference of religion to justify forcing the Armenians to pay higher taxes.
-During WWI, the Ottoman Empire was at war with Serbia. The Serbians were mainly Christian, so the Turks became afraid that the Armenians would defect, join the Serbian army, and fight against the Turks.