Jacob's Enemy is Who?

Jacob has an enemy. A man who finally killed him. That man turned himself into Locke. Who is he? Possibly Christian? Or a Locke who went back and time and came a little to late to the part when Flight 316 came? Or someone more suspicious....... Jacob in the bible has a brother named Esau. Possibly he killed Jacob. Locke is dead, and that sucks really bad. I want to cry but I restrain myself. This Locke imitator can't be anyone we know at the moment, so we have to wait for time to keep up with LOST.