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.
The broadcast time of 'Zhu Xian Qing Yun Zhi 2' had not been confirmed yet. It was expected to be broadcast in the next few years. As for when Zhang Xiaofan had become evil, it was a complicated plot that required more details to answer. The actor for the role of Ghost Li had been announced, but the details of the plot and role changes still needed to be announced officially.
Yes, there are storylines where Doctor Strange takes on a more morally ambiguous or outright evil role due to various circumstances and magical influences.
In fanfiction, when Eren meets Levi in the underground, they might initially have a tense encounter. Levi could be suspicious of Eren's motives. For example, he might question Eren's presence there as the underground is a dangerous and lawless place. Eren, on the other hand, could be in awe of Levi's skills and reputation.
Power hunger could be a motive. Tony already has a lot of power with his technology, but if he becomes obsessed with having more and more, it could lead him to evil. He might start to see others as obstacles to his power gain.