Yeah, those 3 shots he got off were a real mob-stopper !
Yeah. Now imagine a prisoner in a modern prison arming himself with two loaded weapons. A "response" from the prison guards wouldn't be less than an "all-out response." And just because such an armed prisoner, who gets 6 shots off might not be a "real guard-stopper" with such weaponry, doesn't mean that Smith wasn't a real life-and-death threat in that scenario.
Smith had his own large "militia" in Nauvoo among every-day people. Likewise, prison guards weren't a large contingent of people. Their "back-ups" would be the equivalent of a volunteer fire dept in a rural area.
A guard who felt threatened by a prisoner with loaded weapons, with murderous Smith bodyguards on the loose like Porter Rockwell (one who Mormons celebrate his b-day today, I believe)...and a well-armed militia who could act on behalf of Smith...I could at least see why some vigilantes would have felt threatened -- and certainly whatever guards were there could have felt so threatened.