I don't know if it has even been spelled out or not (I think it may have been), but Locke was killed once he spilled the beans on Eloise. Ben wanted to go back; he thought Locke could take him back, but when Locke said Hawking could get him back, he had no further use for John.
Having no further use for Locke doesn't really explain why Ben choked the life out of him. The other time Ben tried to kill Locke, it made sense because he felt his leadership of the Others was threatened after Locke heard Jacob (or whoever the hell it turns out to have actually been). Maybe Ben had a similar reason to not want Locke around anymore once he found out about Mrs. Hawking and realized he didn't need Locke to get back to the Island, but what reason he might have had wasn't made clear. Say what you will about Ben, but he's not the type of guy that kills people for no reason...
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