The malevolent helm lasts for 100,000 charges of combat before degrading to nothing - it cannot be repaired. Once equipped, it instantly becomes untradeable, even if it has not been used in combat.
Losing the malevolent helm by being killed in the Wilderness will result in a percentage of the original malevolent energy being dropped on death, equal to how degraded it was, rounded down. For example, if the helm had 100% of its charges remaining, 14 malevolent energy would be dropped on death, and if it had 52% of its charges remaining, 7 would be dropped. This can be used to refund most of the cost of the helm, although the reinforcing plates are not returned.