Evil Locke is just Locke.

Taken from my post on the ABC forum.

The dead locke and the live locke are just the same person from different times that just happen to be in the same time for now.

If we assume that Locke or anyone cannot exist past their death, then they are free to travel any time before that.
So Locke can live in any time preceding his death (travel in the past) BUT with ONE exception, which is the LOOPHOLE....

If his dead body is present/preserved after his death then an earlier John can exist after the date of his death.

Why? Because destiny requires that he is dead and the dead body fulfills this mandate, thereby creating a loophole where an earlier John can exist after his death. This reason is also why Jacob is also not really dead, IMHO.

This is the loophole.

So, a person traveling in a time after their death, will die. The other possibility besides death is that the person can live after their death, but will be completely incapacitated in some way so they cannot alter their time line. This is why Theresa Spencer is in present day, an invalid, requiring constant home care by her sister. So in this case, my guess is that Faraday needs to bring her dead body back to the present in order to allow her to live a normal life after her death on the island.

If this is the case, then you can change destiny using this one and only loophole.