Talk to everybody in town and eventually Tobias will be implicated. But talk to the butcher about who has shown signs of Venom abuse and he will mention Garritt and give you a key that Garritt dropped near his shop.

**Little Spoiler as of the rest of the quest**

Go to the Fellowship shelter and open his chest (the one with the pipes nearby) to find the missing venom. Return the venom to Morphin and talk to the rest of the involved townsfolk about what you found.