Praziquantel's claim doesn't really make sense. You're willing to let it slide because it makes him unlikely to be a Hippo, but I have seen no real evidence for your Hippos as scum theory.
Stephen had a night kill which you cannot explain. Yunalesca's role message indicated that there was a chance, however slim, that she could turn into a Serial Killer. Both characters were legitimate threats to the town, if there even is a town in this game. I'm not saying that the Hippos are good guys for opposing them, but I also don't think it makes them evil.
At this point, I'd rather try to find a scummer than try to find a Hippo.