As for any investigation, you need to find proof before confronting them. This can be done with at least 2 different way and without leaving town.
In addition, solving undead problem for Aureus yield a ton of xp, and you already did the hardest with the lighthouse fight. With this fight + the xp reward, you should already be level 5.
Did you find the area under the town? There is some level 4 fight and a lot of exploration xp. There is 4 way to get in.
Did you take the pet pal talent? It is the main source of ingame tips and ways to resolve quests.
I tend to be a bit cryptic because I don't want to spoil your game.
