Ben told Locke to kill his dad, because he knew that Locke wasn't able to do it.
Nobody else told him.
Gee I thought Ben told John to kill his father as a test/sacrafice.
hmm
It's possible that both of you are right. Perhaps, for whatever reason, sacrificing your father is something you have to do if you wish to be the leader of the island, but Ben made a point to tell Locke about it in front of everyone because he knew that Locke would back off from the task, and that way he would look foolish in front of the others, his soon to be people.