Ah well. I was just thinking - if Locke is hunting down an element which destroys humans, then that'd fit well with the precursors - they certainly had the means to create elements, and it is not unthinkable that one of their ships survived the (weakened) halo array. Perhaps this element was designed to kill forerunners but instead ended up an anti-human weapon, or perhaps it was intended as a failsafe for if the inheritors of the mantle rebelled as the forerunners did.