If we are talking about actual individuals, there are really only two plausible answers here. The first is Gavrilo Princip. He is the Slavic nationalist who killed the Archduke Franz Ferdinand, thus leading to the outbreak of hostilities. The second is Kaiser Wilhelm II of Germany. He is the one who started Germany's fairly aggressive attempts to get its "place in the sun." His actions helped to cause the overall tensions in Europe that made it easy for the war to explode into something really big after Princip's action.
Of course, there were others who were involved. But in terms of individuals, these are the only two whose actions are well-known and which helped to bring the war about.
Europe that began on 28 July 1914 and lasted until 11 November 1918