Re: Data and killing
Data already has experience of the somewhat arbitrary nature of Starfleet military law, doesn't he? If he'd had an advocate less capable than Picard, he might be gathering dust on a shelf somewhere as the victim of a failed experiment.
If we assume that he in fact believes he behaved correctly, given that his dilemma has been obviated by his rescue, he might simply wish to avoid what he sees as the unnecessary risk of being convicted of a crime he knows was justified in the circumstances.
That's a pretty good read on the situation, I think.
“A life is like a garden. Perfect moments can be had, but not preserved, except in memory. LLAP” — Leonard Nimoy (1931-2015)