Starfleet didn't sanction killing the senator, just forging the record.
Starfleet placed Sisko in a position of authority, in order that he would made decisions. Starfleet is responsible for Sisko's official decisions.
It was Sisko who brought Garek into his plan. Garek, a being with basically non-existent ethics. Sisko, in this particular event
, is responsible for Garek's actions. Garek's focused goal was the end result of Sisko's plan, the forged data rod wasn't going to fool the Romulans, Garek knew this.
The murder of the senator, anyone else aboard his ship, and also the murder of the original forger by Garek can all be laid at the feet of Starfleet. You might, in addition, be able to include any misdeeds carried out by the Romulans during the course of the war as a result of Sisko's plan.
( ... but Sisko only knocked over the first domino ... )
I simply love using "In the Pale Moonlight" when some Trek fans whine about section 31.