It sounds like the original murder was committed by some group that doesn't like foreigners (given the pamphlets). The victim was a dwarf.. can I assume that that's not a native race of Ankh-Morpork? And in game terms, are the natives of Ankh-Morpork the scum? What kind of race are the natives?
The rest of this is just my opinion from what little I know about the series so far:
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Now the pamphlets... there seem like there are 3 types. First, the New Hippo League: sounds like they would be responsible since they don't like foreigners and there was a hippo branded on the victim. Then there is also the case of the building being smashed.. possibly by a mob with a hippo-tipped battering ram (although that isn't confirmed). I wonder if the two events are really connected or if they were done by two separate groups, with one pretending to be the New Hippo League. This might make sense with the Phantom stuff that's been appearing.
Next, the Undead. I don't know much about them lore wise, but it sounds like the group might be natives of Ankh-Morpork as well. Artsapat suggested they could be a cult... does it make sense? Can they forcibly turn people into Undead, like with magic or something? In any case, there's nothing yet to suggest this group is responsible for the death of the dwarf. Are their numbers large enough to form a mob to destroy a building? I'm going to guess no to that as well, but feel free to point out if I'm wrong. So if they aren't responsible for the death of the dwarf or the destruction of the building, but are still considered natives of Ankh-Morpork, I would guess this group is some kind of neutral non-killing faction. Probably not a cult (unless they have some recruiting mechanism story-wise that I'm not sure of), but possibly survivors, jesters, executioners, etc.
Finally the Om.. now this sounds like a typical religious cult. They could be a cult, or maybe the mob that destroyed the building, and pinning the blame on the Hippo league. I don't know if this group are natives of Ankh-Morpork or foreigners, though.
So, in conclusion, my guesses of the pamphlet factions are:
New Hippo League: classic mafia (killing faction)
Undead: benign/non-killing neutrals
Om: cultists (non-killing faction)
As for the event, I did notice it in time so I got my choice in. Should we go about sharing this info? Isaac said the events matter but not as much as playing the game normally. Sounds like it wouldn't hurt to share this info, then.
I'll go ahead and say I picked Option 5 and I got a small reward. Nothing really game changing though.