If the blocker was following orders and believing he was blocking not to prevent someone from doing something illegal, but just in the belief, contrary to theirs, that they'd die if they went forward but are safe if they stay, would in a free country he be within his rights to warn them that they were taking their lives into their own hands by going forward, he didn't advise it and believed his information was superior to theirs, but that he couldn't/wouldn't stop them if they insisted on going to their otherwise legal route?
Those are great questions. Wish I knew the answers.
All I do know is what I’d have done: I would have treated it as a self defense situation and dealt with legal ramifications afterwards.