I agree that Fel was killed because he knew how to seal the Dark One in his prison, if not for forever, at least for many Ages to come. He suggested that Dark One has had to have been away before. The Wheel of Time turns, and Ages are eventually repeated. Each time, the DO is breaking loose, and each time he is sealed away. But Fel thought that for a long time now, the DO has been breaking through the patches. He breaks through the original seal, they put a patch over it. He breaks through the patch next time, and they put a patch over the patch. Each time it is easier for the DO to break through. Fel said that the rubble first has to be cleared away. This provides a clear surface for the new patch to take hold of. Thus the seals must be broken before the DO can be stopped. Even Lews Therin in his mad ravings knows this. When Rand was given the seal after Moiraine fell through the "door", Lews was screaming and trying to break it so it would be done wit! h. (I am doing this from the top of my head so I do not have exact references for you.) Fel already gave the vital information, for him to remain alive to explain it would ruin the mystisism RJ is so good at creating.