As I, FT and others already pointed out, Stephen's death made absolutely no sense. Even if Stephen was targeted, any anti-Vampire / anti-foreigner faction would've been better off using the NK on Yuna, who was also revealed as Vampire at that time. As FT said, and I also already mentioned yesterDay I think, that would've lead to at least one, and if they're lucky (no replacement could be found, and/or Yuna had the wrong sort of information to replace), two less vampires.

But what if Stephen wasn't the target at all? With the large number of unusual roles, having one redirector or whatever counts as a bus driver in this town, would not be a very large leap. In-game, magicians would be able to do this pretty easily (screwing with the mind of one player to make them target another), maybe the Patrician (ordering them somewhere else)? In the case of a redirector, it might be a random redirect (which ended up with Stephen), or it may even be deliberate. Substituting Stephen for someone else could make sense for a pro-town player: whatever nefarious deeds someone would have done to another player is now directed to a man with a death warrant.

In the case of a redirector: if you know who you targeted during that Night, you know who killed Stephen and is thus a candidate for Lynching. In the case of a bus driver, it's likely more to protect someone else, who was an obvious target.

With the manner of Stephen's death, which indicated to me something REALLY large falling on top of several buildings, I don't get a very "normal" scum faction feel. More of a "vengeful god" or "supernatural being" feel. There is also the "Hogs of War" graffiti, and I can't shake the feeling of a vigilante superpowered creature. (Might be confirmation bias from my Hogfather association.)
If the Hippo faction exists (and again: besides several indications neither an Undead faction nor a Hippo faction has been revealed - only a Watch faction) I thought this would include normal Ankh-Morporkians (as far as "normal" applies of course), not a supernatural being.