I do not understand how he fits into the equation. He won't tell anything to Gabriel except for "Cabrit Sans Cor'." That and the fact that his store was one of the first 2 voodoo places Grace found for Gabriel made Willy Walker the main suspect to me when I first played the game. Now that I've beaten the game many times, I'm not sure he had anything to do with the murders. But then why is he so secretive about what he wants to tell Gabriel? Even Dr. John was willing to share some information about Voodoo with Gabriel, and Dr. John clearly has incentive not to. How come Willy Walker is more secretive than Dr. John? That makes zero sense. There must be something I'm missing. And even though I don't think I saw him at the swamp or the hounfour, I think Willy does know something about the murders or he wouldn't have said "Cabrit Sans Cor'" when Gabriel brought it up.

He practiced Voodoo and didn't want to talk about it to outsiders. There was nothing in the story which would indicate that he was part of Tetelo's cult as there were many other Voodoo practitioners in town. Those who practiced light Voodoo didn't want to get involved with the affairs of dark Voodoo practitioners.

Willy was secretive about the Voodoo cult for the same reason that Crash was: Tetelo, Damballah, and Ogoun Badagris scared the crap out of him.

You can see this most clearly if you show him the photo of the murder victim rather than just ask him about the Voodoo Murders. He'll tell you about cabrit sans cor' either way, but he's clearly more rattled in the former case.