She's the single most close-minded character in the entire series and it led her to murder one of her friends and attempt to murder another because she couldn't handle the truth she'd already previously been told. And she's just as close-minded in Rebellion where she flat out refuses to even entertain the idea of what Homura, one of her good friends, is telling her.
Mami's character is as complicated as all the other characters imo. I would argue that her actions at that moment were due to panic, stress, and shock. Even though her actions were drastic, dying would be the only way to be free from the curse of being a witch. I don't see it as if she couldn't handle the truth, she just jumped to a conclusion on shaky circumstances and decided to recklessly act on them. She is just a middle schooler after all.
I feel as though her actions are all believable and within her character, due to her experiences before the events of the main story. It's probably because I've read The Different Story, but I actually feel a bit sorry for her. It's been years though and I'm not sure if I'm remembering everything correctly. I guess it's a good time to reread it to make sure I'm not just being a Mami sympathizer lol.