What causes death in african sleeping sickness?

Brain damage: The immune response to the organisms in the brain is extreme. There's even a preponderance of the otherwise-uncommon "mott" plasma cell in the brain. Please get treated if you are affected, even though the treatment itself is unpleasant and dangerous.

Coma: The t. Brucei organisms can cross the blood-brain barrier leading to confusion, severe fatigue, sleep reversal, reduced coordination, coma & death.