Mikasa was on edge because she knows that Bertholdt is dangerous.
Mikasa was on edge because Armin is her family.
Mikasa immediately lunged for Bertholdt because if she hadn’t, there was a high chance that Bertholdt could’ve hurt or killed Armin.
We already know from many chapters ago that Mikasa prioritizes Eren and Armin above everyone else, and with good reason: they are her family, and she doesn’t want to lose her family ever again. She’s already lost them twice and has nearly lost Eren and Armin multiple times. She’s completely justified in being simultaneously protective of them and heartless toward opponents she sees as threats to her family’s peace.
That being said, she still cares for her other comrades and friends. This is implicitly evident in Mikasa noticing the change in Bertholdt’s personality. She may not say much, but she’s observant and perceptive when it comes to people who matter to her, and at one point, even Bertholdt was her comrade.
Mikasa did not fuck up the mission. Rather, she protected Armin and ensured that he didn’t do anything stupid. Armin did not call her a “stupid bitch” at all in the chapter, and I have no idea why anyone thought so. Armin was thankful that he saved her and thankful that she stayed safe.
If anyone dies as a result of this mission, it’ll be because Reiner and Bertholdt have lost and any all mercy they once had toward their former comrades, and it will not be because Mikasa couldn’t kill Bertholdt. And for fuck’s sake, if she did kill Bertholdt you all would be hurling misogynistic slurs and hating on her anyway. Stop with the cognitive dissonance.
Why is it that when Levi failed to kill Bertholdt, people rushed to his defense but when Mikasa fails to kill him people just criticize her? Why is that even though Levi and Mikasa are both referred to as humanity’s strongest and even though they’re most likely related and have very similar fighting stiles, people glorify Levi while calling Mikasa “overpowered” or “overrated”? Why is it that people simultaneously and erroneously say that Mikasa “only cares about Eren” but the minute she rushes to protect Armin you all call her a “bitch” for hurting Bertholdt? Why is that you’ll call Mikasa a “bitch” for attacking Bertholdt to protect Armin when you all railed against Reiner and Bertholdt for killing Marco when chapter 77 came out? Why is it that you’ll forgive Reiner and Bertholdt literally any other time but you’ll villainize Mikasa for doing the right thing and protecting the only family she has left?
I know why. I think the fandom’s reaction to chapter 78 is sufficient enough to shut up any claim that the fandom isn’t misogynistic toward Mikasa. :)