Locke is they key to everything. From Season 1-5 the Nemesis has been trying to find a loophole in someone on 815 to kill Jacob, and in Season 5 he found Locke!
Nemesis: "You're trying to prove me wrong" Jacob: "You are wrong"
They're not talking about showing the goodness in mankind, but that the Nemesis will never find a perfect loophole and kill him. And when Jacob said "They're comiing, he was refering to the pople in 1977, because they DID restart everything...mabye not exactly as they thought, but they did restart it to before the Nemesis killed Jacob...that's why he looked so ticked, becuase now he has to re-do his plan with Locke.
There's my throry. It's just the NEmesis trying to kill Jacob, but 815 poeple were the first to mess it up and actually CHNAGE it. So there is a master plan, and it DOES involve the 815 people in it.