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.
Well, Eren Jaeger's transformation into the monster of the story is a complex process. He witnessed so much pain and suffering, especially the loss of his loved ones. This, combined with the power he gained, made him believe that only by using extreme force could he achieve his goals. He was no longer the naive boy he once was, but a force of destruction, ready to sacrifice countless lives for his vision of freedom, which ultimately made him seem like a monster in the eyes of many.
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.