Jacob's touch only prevents from suicide, for some reason.
This makes no sense, since Locke didn't commit suicide. He was pushed out of the window.
Locke was later killed by Ben.
There is no definite proof that Jacob's touch makes you immortal. Richard could easily have been speaking metaphorically, or there could be some set of rules to the touch. I don't think that Jacob is the king midas of immortality, and Locke dying is evidence that he isn't.
Locke was immortal on the island also (he survived a gunshot wound), but Ben strangled him off the island. He was the only character that Jacob touched that was killed off the island.