Why is Data ok with killing borg in First Contact?

[at Quark's]

Because the Federation had made too many compromises already. Too many retreats. The Borg invaded Federation space, and the Federation fell back. The Borg assimilated entire worlds, and the Federation fell back. Not again. The line had to be drawn there. No further. And Data was going to make them pay for what they had done.

Also because he wasted way, WAY too much time on those Borg already, thinking about their tempting offer. For an android, 0.68 seconds is nearly an eternity.

 

[Farscape One]

I think it was simply a matter of him performing his duty, because the Borg were not just a threat to the ship and crew, but the timeline, as well.

And besides, Data was doing those drones a favor, which Picard rightfully said during his briefing with the security officers. If I was assimilated, I would hope for that favor.