Tireless search of the one true cause has never been part of serious historical inquiry and studies. Especially when you claw your way so far back into the past. Maybe, maybe not.

You could say he was an indirect cause, in that by unifying Germany he set the stage for WW1. However, many would argue that it was Germany's abandonment of his policies under Wilhelm II that was a far more direct cause. Bismarck generally sought to stay friendly with Russia, not needlessly antagonize Britain, and otherwise not "poke the hornet's nest" with other powers. (The annexation of Alsace/Lorraine was the General Staff's idea, not his.) He realized that, as powerful as Germany was, they should not try to fight the rest of Europe. The major blunders occurred after his fall from power.