(response is based on the 1930 version of the film)
In the film version of All Quiet on the Western Front, Joseph Behm is directly responsible for his own death (unless of course, one is looking for a literal answer--in that case the opposing forces are responsible for his death because he was shot by the soldiers on the opposite side of the front). While the company is moving along the front, Behm is shot and shards from the spray blind both his eyes. He goes a bit mad when he is blinded, and rather than lying flat and waiting for help, he stands upright and runs over and out of the trench. Because he is the only soldier visible to the opposing side, he is shot numerous times and falls. One of Behm's fellow soldiers disobeys Kat's orders to stay low and runs onto the field to pull Behm back into the trench. By the time he gets back, Behm is already dead.
Figuratively speaking, Behm was killed by the devastating force behind the ideology of war--Behm did not want to enlist as a soldier, but he was pressured by his school master Kantorek to be a patriot and fight for the "fatherland." It is ironic that the soldier who did not want to enlist was the first to die on the field.