This really bugs me, honestly.
If we walked in on a guy laying dead on the floor from gunshots, a pistol discarded across the room, then maybe I could buy the Letuce siblings were responsible.
But, i just can't see either of them tying a guy up, putting a bag over his head, and then going after him with a knife and a chisel. That's some cold-blooded Saw-type shit. I could see them doing something quick and dirty because they know they *have* to, but I can't see them doing something so...deliberately malevolent.
I don't really see any other explanations for how the lighthouse keeper ends up dead, though, so I have to settle that the siblings murdered him just... gruesomely. It really kind of messes up their characters a bit, for me.