Armenia was sucked into the Ottoman Empire in the 15th century. The Armenians were given lower rights and had to pay higher tax's than Muslims. Throughout the 19th century though, Armenians prospered under Ottoman rule.
The Ottoman Empire was not pleased with this. The Armenians were mainly following Christian laws and teaching. The Ottomans were also not pleased with that. At the end of WWII the leader of the Ottoman Empire ,(Sulton) was obsessed with loyalty at the end of Ottoman Empires reign.
So to "insure" that everyone was loyal to him he in acted a plan to deal with the entire Armenian population. This plan would consist of fire squads, concentration camps, and later, death marches in the desert.