How did Pelzer die?

Pelzer was tied to a tree and tortured to death... he was murdered.

On his face too there was a gleam of that light compared with which Overkamp's torture lamp was but a miserable contraption. Pelzer had closed his eyes; his face had lost all its delicacy, all its timidity and weakness; it had grown bold and sharp. His thoughts were collected; they did not dwell on doubts and evasions, but on the comprehension of the inevitable.