By the time he was assassinated, the French government (Vivani and Poincare, anyway) had already agreed to assist the Russians if they were attacked by Germany and Austria-Hungary. He isn't going to keep France out of the war.
What he could do, however, is provide a strong leader to the movement to get out of the war. As it was, France's three main anti-war politicians were either dead (Jaures), exiled, or discredited (Caillaux) due to a scandal involving his wife. With Jaures on board, he could provide a potential rallying point. Whether it would be sufficient to get France out is another matter. As Becker has indicated, apart from a brief period in early 1917, the French people, no matter how heavy the casualties or material shortages, remained more or less committed to driving the German invaders out before considering a peace.