Early on he wanted someone else to be the Big Bad, but when editorial said no, he quickly shifted to Moira being a bad guy. I mean, the entire Inferno book was about how Moira's backup plan was to depower everyone. She was clearly meant to be the antagonist by that point.

The rest of her arc -- becoming a robot and afterwards -- wasn't written by Hickman, but it was discussed with him. I think they said he was in the room as they talked about the next two years of stories after Inferno.