"Annie showed no guilt or remorse for all people she killed and she enjoyed doing it, unlike Reiner and Bertholdt"
Sure man. That's why Annie was the first warrior to say sorry. That's why she jumped at the chance to be seen as a good person by Armin. And certainly why she had a 1000 yard stare and apologized while seeing the aftermath of Trost, cried in the forest after killing so many for no gain, was horrified and cried when Reiner pressured her to kill Marco, and awoke shaken by PTSD nightmares of her actions.
She must just be a heartless bitch, right? That's why she killed people with such cold effiency during the Female Titan Arc. Definitely not because she forces herself into a detached state to avoid the emotional torment of her actions. No no, that can't be it. We all know she's a one-dimensional blood-thirsty beast who finds glee in yo-yo-ing the people she's grown to care for despite the walls she built around her heart.
I have a theory that many people have simply forgotten the depth of Annie's character because it's been so long since they've watched season 1; back when they may have had a much more black and white view of Attack on Titan's central conflict. Reiner's most sympathetic scenes were portrayed more recently in season 4, and by that point, viewers have been made aware of the full, nuanced picture of Marley and the plight of Eldians. Whereas Annie's most sympathetic scenes were back in season 1, and in season 4 she says, "I'd do it all over again." Pretty damning statement, right?
Except, that sounds strikingly similar to fan favorite Levi's philosophy of embracing the consequences of his decisions without regret. Interpreting Annie's statement to mean she's heartless disregards the context she was raised in. Just before that statement, we're shown her backstory scenes which portray how Annie was raised to not value any life, including her own, resulting in a nihilistic perspective that informed how she conducted herself among peers and in battle. Annie knows she would not conduct herself any differently if she could do it all again because the environment in which she was raised would produce the same outcome and the same choices. Just like Levi, Annie made the "best/logical" decisions she could in each moment, and finds solace in understanding those choices, staying true to her goals, and accepting the blow-black.
But Annie isn't made of stone either. She does feel remorse. She bears the pain of every person she's killed. Every moment she could have resisted the current and made a selfless stand. And so, like Levi, Annie tries to go day by day making choices she won't regret. Trying to lean into the "good" Armin saw in her all those years ago.
As soon as she realized she had the potential to be something beyond a weapon of destruction - to be human - Annie's walls began to thaw. Leading to the Annie who swooped into the final battle, regardless of personal gain or injury, and provided clutch support in saving the remnants of humanity.
