Since it is established in TNG canon that the Borg destroyed Guinan's homeworld, it would seem to have made sense to have her in a story where the Borg are so heavily featured. Also, she would have been terrific in her well-worn role as Picard's confidant/advisor - especially in terms of how he was dealing with Worf and Lily.
Here's the thing, though: They already dredged through these issues in "I, Borg". That's one of the many things that makes FC hard to watch. Picard dealt with his feelings about the Borg. He came to terms with them, and in the end he took the civilized route. So, which part are we supposed to believe? That seasons 4-7 didn't happen? Or that Picard's judgement in I, Borg was wrong?
BINGO, he made the wrong choice and and he regrets it. Considering how many people probably died during the Borg's latest assault and the mess they made of history, I don't blame him.
We've made too many compromises already.Too many retreats.They invade our space and we fall back. They assimilate entire worlds, and we fall back.Not again!
He's angry that he comprised his resolve by saving the life of one Borg and now he's wants to make sure he doesn't let his conscious get in the way again.