I can prove why he would have lied.
Ever since that huge arguement over wether Data was a sentient being or Starfleet property,people would have wanted a reason to have Data shut down.
He did not want to be disassembled in the first place but to kill Fajo would have gone aganst what everyone percives as one of his limitations.
To go over one of his limitations would give Starfleet an oppertunity to claim that as a malfunction.
Why shut down Data? To study him and make more Soong type androids.
Then why, did he say, right before he was transported, before he was going to 'supposedly' fire, as Fajo was giving his speech about how Data could not feel revenge, that he simply would find someone else to hurt or kill so that Data would obey, with Data looking down, he said:
"I cannot permit this to continue."
That seems pretty final to me. The weapon did discharge during transport.
When I watched it, I thought Data would do it. It was a justifable action; to prevent further harm to others, the only way he could end that would be to kill Fajo, or himself.