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Even if Ed decided to go through with it, he would probably have backup from Central too, who had taken control of Briggs on Bradley's orders, and maybe even the homunculi because there is no reason for them to hide in the shadows anymore. They pulled the same stunt with Mustang to attempt to get him to do human trasmutation by putting Hawkeye in danger. The homunculi's fatal flaw is that they underestimate humans, which is what leads to them being outsmarted several times, so they probably thought Ed would cave as easily as Marcoh did if given proper incentive.
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Note that Pride was begging for his life when Ed destroyed his Philosopher's Stone, and Kimblee had to correct him, saying Ed wasn't that kind of person.
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The only explanation I can come up with for this is that Awakawa needed the Elric Brothers to be in this part of the story and didnt want the antagonists to have a massive advantage (though they wouldnt be winning regardless of what happened at Briggs because of Hohenheim's plan involving the reverse transmutation circle).If Edward Elric DID try to kill the soldiers of Briggs he couldve easily been killed himself and Father wouldve lost one of his sacrifices.) (Not to mention, that to Father and the military higher-ups, Kimblee was expendable as he hadnt opened the Gate and they did not believe he would/could. The homunculi had Kimblee, who could and DID carve the crest of blood anyway, so why did they send Kimblee to try to convince Ed to kill people rather than just sending Kimblee to Briggs to look for Scar and then have HIM carve the crest of blood? Involving Ed just seems like needlessly throwing a wrench into their plan.Pride couldve spied on the Elric Brothers while they were in Central, and Envy saw firsthand that Ed was apprehensive about fighting the "people" on his monstrous form and using the Philosopher's Stone, which should have proven to him that Ed probably didn't have the mind to commit mass murder.
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