Locke saw the light by Jasmine Lovage

Locke said to Jack that he had seen the eye of the island and it was beautiful......

i think he had stumbled across the cave, and possibly knew what it was, or just instinctively knew it was good!

he also came face to face with Smoke, i believe that the smoke read him, and knew he had seen the cave and that is why he spared him, then MIBs plan began to gain access to Lockes form and his memory of where the cave was!

we now know that MIB was the voice in the hut in season 3, and we also know that he posed as Christian in order to get Locke to leave the island, where Ben killed him and Eloise insisted that his body be taken back to the island..... these were manipulations of MIB! he wanted access to Lockes memories!

Charles tested Desmond and during this experience Desmond became aware of the sideways reality, he must have known what it was.

MIB threw Desmond down the well and later sent Sayid to shoot him, i believe that what Desmond told Sayid was that there was something more after death, and he would find his love there when the time was right! so Sayid spared him.

MIB still needed a way to destroy the light...... i believe that what Charles Widmore whispered in his ear was that Desmond was the key to this..... that he was the only one who could safely approach it!