I guess it could be done without too much railroading. You just have to make the decisions plausible enough for the players, so they decide to kill the werewolf and come with the guards.
Start off with the PCs facing the werewolf, make sure during the battle, that the werewolf is going to get killed. If they are trying to subdue him, they should get into trouble quickly (just make the werewolf tough enough).
Or start the fight, with the PCs already wounded, right in the middle of combat, maybe they have struck one or two wounds to the werewolf already. Setting the right mood this way. It should evolve into a him or us situation.
Once they have killed him, there are sounds in the ally behind them, and they hear a woman scream, the militia arrives shortly thereafter.
Full knowing, that they have done nothing wrong, they go with them probably (the guards won't call them murderers, just politely ask them to come with them), and then you can continue with your story and let the campaign begin 6 months later.
Bye
Thanee