Here's what I don't get about Fajo's energy field. Couldn't Data have just thrown something at him, or used a good ol pointed stick? If the smoke monster could figure that out so could Data.
Wouldn't killing Fajo have been an act of defending others, had the Enterprise not figured out he was still alive? Fajo was not only a mass murderer, he was a slave driver who would, for sure, murder and enslave more innocent people in the future.
I think Fajo saw Data as just a robot like we see the little Japanese robot dogs. He didn't think Data was capable of forming original thoughts.
Starfleet has never been Pacifist. They do, however, strongly prefer peace to conflict, to such an extent that they are over-eager to make ridiculous concessions to avoid war. Data's the same, and Fajo put him in a position where the moral cost of letting him live exceeded the moral cost of killing him.