I believe the Exo are going to be the resurrected dead. Humanity had its back against a wall, they needed more man-power, so the next logical choice was to take the dead, dying or volunteers and turn them into men (and woman) of metal and electricity to fight anew. The dark and sinister-ness comes from the fact that they are cybernetic undead, fighting with the zeal of people born again. Their purpose is to destroy the enemies of man-kind with extreme prejudice. However... they seem conflicted. Questions of who... what am I? The image I provided evokes feelings of focus, rest, but he also seems transfixed and concerned. It's amazing how a metal face can still show emotion. But, in the end, regardless of what they become they will forever be remembered as heroes who made the ultimate sacrifice and lived to pay it again and again. They will never be forgotten, their names etched in the halls of history, hallowed in time. It's almost beautiful when you think about it.