If battle table PvP is available in Expo, it would be a supreme p2w opportunity. It is entirely possible that individual or small-group PvP would be allowed, but battle table PvP would be forbidden. Or perhaps only nobility (i.e., those actually holding land with noble titles) would be able to engage in battle table PvP during that period. Nobility by definition would not be landless.
However, even if the landless army concept is not entirely feasible, nobles not part of a kingdom's initial founding cabal or nobles betraying that cabal are near certainties. The feasibility of siege-capable armies in Expo or at launch thus becomes a question of the logistics of getting all the participants to the party, just as would be the case for a landless army. The feat may be easier, because some settlements can be counted upon for basing and support.
This is a digression from the original topic, because the notion of even a landless army implies logistics. In order for a group of raiders without logistics to kill every character in a settlement, I think the settlement would have to be undefended. Any settlement with walls and gates will present an almost impenetrable obstacle to raiders without siege equipment, unless the AI for NPC and OPC defenders is effectively nonexistent. I say this based on the published statement that sieges with the proper equipment will take RL hours to days, rather than minutes.
Trying to get back to the original topic, obvious malfeasance by deviant characters is likely to meet active opposition from conscious NPCs, and by extension conscious OPCs who should be able to run basic reactive scripts the same as NPCs. So it is possible for a particularly competent assassin to kill several people in a settlement, but it is not so likely that settlements larger than villages will be easily massacred by groups using brute-force tactics. The OP's concern is valid, and it is a good question, but the event should not be likely in a well-run settlement, even a hamlet.