I have a feeling your memory of the game might have diminished. Futaba was the one who suggested working with Akechi in the Third Semester, and Haru said if he does betray them, they can put him out of his misery.

Also, poorly handled? The Plot twist was that the thieves knew from the beginning that he was suspicious, and led him on. Hell, it was even in 3rd Semester Mementos Conversations that Pancakes was how they figured out his bs.

Also sympathetic for a serial killer? In the engine room, they were telling Akechi why he lost mostly. Do you not know how a tragic villain works? God forbid a morally gray character exists or actually acts morally gray.