Good question. But looks like F4 was there to prevent the use of the gas because if Ben was to activate before freighter guys get to him, everyone including Freighter people on island (brainy 4 or the armed guys- don't matter) would get killed. So it's a safety measure for Freighter people themselves before anything else..But I do agree that not everyone on boat was aware of the killing mission..

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"Bear...? Is that you?"-- Hurley S03E4