I'm doing this from the top of my head, without re-watching the episode(s) in question.
Killing the senior staff would be easy, except for the timing. If they all were murdered early on in the mission, it would raise holy heck. Obviously the work of a Dominion agent, and since it would be difficult for a jem'hadar to hide in the ventilation shafts of DS9, everyone would suspect a changeling. A changeling running loose would raise paranoia across the quadrant, lock the station down on double-secret red alert and generally make the Great Sun Flare Mission far more difficult.
Killing the senior staff closer to the checkmate move of the Flare event shows an astounding lack of confidence in the entire grand strategy. DS9 and three fleets would all be killed in one stroke. Why bother?
Perhaps in a last microsecond "d'Oh!" before being vaporized, the changeling had this exact regret.