I feel it was revenge. If it was just plain logic why did he lie about the phaser at the end of the episode?
That he lied doesn't dictate that he must have done so as a coverup for revenge. Sure it's a remote possibility, but given Data's entire history, the least likely one. It's far more likely he lied, because the truth could be interpreted in a way, that wouldn't be in his best interest, even as Fajo was a clean kill, by my account, since he was still in command of his ship, & still a threat to lives.
Without being too indelicate, Data suffers from Starfleet's equivalent of Jim Crow. He is an oppressed individual. Whenever the issue of his liberty or his equality comes up, the knee jerk default is to deny him. Refusing an experiment? No. You're a toaster, until you fight for it. Have a daughter? Nope. We'll handle that. Want a promotion? Nah. Not even when everyone else is getting them. Even your greatest advocate Picard will have to be reminded, by you, that you merit a command, when it comes up in the course of duty (
Redemption 2)
He'll even remind you of your subjugated status, if another situation arises that calls your dependability into doubt (Clues) I'm not saying Picard was personally threatening Data with being stripped down to his wires. He's only warning him that's what
others in Starfleet would likely do, & he was accurate to point it out.
Taking all that into account, Data has more reason than anybody to be suspicious of Starfleet treating him fairly, because they never do. Now consider Fajo. He was killing him, & the reason for it was just imho. However, the scene explains that in order to do so he must disregard a directive he has about killing. If that directive or program (or whatever) was known to Starfleet, & it then became known that he disregarded it, is it not a possibility that such being known might bring into question what they trust about Data?
Does Data need that potentially happening AGAIN? Starfleet prodding into what they trust about Data, when they ALREADY don't trust him to be a captain, a guardian, or even a free individual at one point, has catastrophic possibilities for him. Frankly, being able to kill Fajo just puts Data on the same footing as any other person in that situation, but that he isn't supposed to be on the same footing with everyone else can too easily be held against him by the people in charge, like it always has been.
It's better they not know, & Data knows that, especially since it didn't come to that anyhow, & it wasn't in the line of duty either. It was personal. So Riker & anybody he'd be reporting to about it have no specific grounds to push for an answer, which is probably why no one does. It nobody's damn business, & what good could come of it? and that's how I've always interpreted Data's refusal to offer the whole truth