Safety spoilers
I feel like we were forced to kill him because there was no guarantees he wouldn't start pulling more stunts to get our attention again if we chose to ignore him. Though even if he did get what he wanted in the end, if it made him come to a painful realization of how utterly empty his existence was compared to ours, that could arguably be considered more cathartic then simply denying him that truth. The fact that he even put any thought into Alisaie's words and asks whether your own existence satisfies you makes me feel like he did finally make that realization despite his answer being left vague.
I was never really expecting him to get much development past a true understanding of what makes us tick, and that final solo fight to close off the story felt like the best way they could've handled it.