Appropriate to you perhaps but friendship is over as soon as you kill scores of my comrades including new friends I've made. He seemed to have completely forgotten what set him off and made him turn into a titan and fight her the first time. She's shown no consideration for human life and even then when he's being a little bitch she's striking blows to the tunnel that could potentially kill his two besties in the world. Sorry but he's being a bitch of Shinji proportions.
He hasn't forgotten, in fact remembering makes it harder for him now that he knows it's Annie. Before, he didn't know who it was, so the fact that she killed his comrades made it easier for him to fight her. But now that he knows it's Annie, he hesitates because this is someone he has known for a few years and is supposed to trust, and Armin/Mikasa had no hard evidence it was her in the first place, and he didn't want to believe it could be her. But he finds out it is, and that makes the decision harder. Yeah, it
should be easy regardless because of how many people she's killed, and the fact that she could kill Armin/Mikasa/anyone else while he's deliberating, but there's a lot more going on internally that gives him hesitation. Not the least of which, I'm sure, includes the fact that human or titan, Annie has always been superior in combat to him, which could possibly psych him out a bit too. But mostly that he has to fight the idea that Annie is against humanity.
And she has shown some consideration to human life, she spared Armin, which was kind of big. I have to agree about the Shinji comparisons though lol, it may just be exaggerated for the anime but he may as well be Shinji Jaeger.