Acting out of character is pretty subjective. People in the real world change over time. Places where you start with hope and equanimity can change to places you have to shrug your shoulders and just call it a day. This actually happened with my job in the last couple weeks, and I am now looking for a new one even though I love a lot of people there.
When I think of Guinan, I think of someone who is preternaturally calm, sure. But she was already shown to have a temper. She stabbed human Q in the hand after all.
And we don't know how Guinan changed over 400 years. She seemed very young in 1893. Is she on the path she would have been if the "Time's Arrow" events had happened? A lot can change. In this new version of the 19th and 20th centuries, did the Devidians ever stop? Could she have lost a lot of her foundation on Earth as they devastated the population? Without the glimpse of the future, she could have over time come to the conclusion that the path of the planet was more-or-less doomed. Maybe she asked for the Aegis to get more involved, and was rebuffed. Maybe she considers herself a failure because her ability to listen was not enough to make much impact on a world heading in a bad direction. Any number of possibilities that have brought her to the place she is, where some random guy shows up the day she's closing up shop, giving her dog away, and probably leaving the planet and tells her she's his only hope while being super cagey.
