Well, it all happened when Eren got hold of that special serum. When he used it, it activated a genetic trait within him, allowing him to transform into a Titan. It was a crucial moment in the story that changed everything for him.
Mikasa loves Eren without a doubt. This is evident in various scenes and her constant concern and protectiveness towards him. It's a key aspect of their relationship in the manga.
Yes. This event occurs as a significant plot development and has a major impact on the story. Mikasa's action is driven by complex emotions and circumstances within the narrative.