
It seems like he didn't have access to his memories or feelings from when he was in Hikaru's body, he only knew there was a wish for him to fulfil, without any conscience or consideration, just the way he used to grant wishes before obtaining a human body. He didn't even seem to remember Yoshiki before returning to Hikaru's body.

that's what i thought too, he's not fully conscious without the body/brain, since he's just a soul/unknown entity without it. what he was doing is understandable bcs he was even about to consume yoshiki (someone he loved the most, and the very last person he wants to kill) if it's not bcs of that stone protecting yoshiki's soul. he needs to fulfill the wish but the only way to go to the other side is to eat the soul connected to him, and when he saw a chance (that girl's soul) he grabbed it without thinking bcs he didn't have any awareness as a mere soul.
Aaaaaaahhh they mergeddd, the new form is so coolll!!