Whilst he did technically tormented Shizue, he didn't actually intend to make her suffer in the first place. Leon himself was formerly a human, though sometimes he doesn't take into consideration the consequences of his own actions. As explained in Veldora's diary as an anime special, Ifrit (the spirit that Leon summoned for Shizue) wanted to serve Leon and not to serve a little kid, that conflicting mindset then made Shizue felt pain and sometimes not in full control of her powers because the spirit that's possessing her (Ifrit) didn't fully dedicate himself to "I want to see this child live a happy life" type of ordeal.