'hikaru' as a being that a human cannot understand is simply doesn't have any 'human memory/awareness/morals/consciousness' without the human body/brain, since he's just a soul/unknown entity that doesn't even understand the difference between life and death without it. what he was doing—as a mere unknown entity that is not tied by any human moral and norm, is quite understandable bcs in that state he was even about to consume yoshiki (someone he loved the most, and the very last person he wants to kill when he's in the human body) if it's not bcs of that stone protecting yoshiki's soul. it was all bcs he—as that unknown being, 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 (the girl's soul that made a connection with him) he consumed it without thinking bcs he didn't have any human awareness as a mere soul.
Ty for this! You explained it really well
HUH DID HE TAKE HER SOUL