The romantic feelings Eren had for Mikasa felt very sudden to me. From my perspective, it was clear that Eren deeply cared about and treasured Mikasa, but the anime mostly portrayed his affection as a family bond. So when the romantic aspect was revealed, I was left wondering, "Wait, when did Eren actually fall in love with Mikasa?"
As for Eren becoming the villain and his friends having to save the world by killing their dearest friend, I'm still not sure how I feel about it. Throughout the story, there seemed to be only two proposed solutions: Zeke's euthanasia plan or Eren's Rumbling. Eren chose the Rumbling, but in the end, the outcome wasn't all that different. Mikasa eventually lost Eren and had to continue living without him, whereas she might have been able to live peacefully with him under different circumstances. (which was the IF route shown if they had escaped)
I don't know. I feel like there isn't really a "correct" answer regarding the choices everyone made. That's part of what makes the ending so tragic and open to interpretation.
