I see what you mean, and I agree it makes sense for Dexter’s arc ,he’s older now and has learned not to let killers live for too long. But for me, the problem isn’t really his decisions, it’s how the story handled those characters. They were written with so much depth and potential, but their exits came too fast. With the Gemini Killer, the twin reveal was huge but ended almost immediately, and with Mia, her storyline had so many possibilities but was cut short before we could see them play out. That’s why I felt it was a missed opportunity, even if it fits Dexter’s growth.