One thing I was wondering is why does Borgman hesitates before he commits certain negative actions? The first was with the gardener. He goes to use the blow gun, then he sort of stops and thinks what he is doing. I thought he was going to have a change of heart in that moment.
Another is when poisoning Marina. Before grabbing both white glasses, he seems to sort of ponder about it for a moment.
For a time, I assumed that Borgman, as the (a?) devil, was restricted by some sort of cosmic rule that prevented him from materially causing the death of humans. He could harm them (e.g. poisoning to cause sickness) but not directly kill them. But that theory finally seemed to fall apart when he killed the husband by poisoning.
RE: The gardener: He was merely waiting for the right shot. The gardener was bent over, with little skin exposed. Borgman needed skin, as even one layer of clothing could conceivably prevent a blow dart from penetrating.
RE: The wine: I took that as Borgman's one moment of almost making a mistake. He had poisoned her wine, but forgot which was which. When he crossed his arms to pick the glasses up, he had it figured out and gave her the poisoned wine. Nothing more than a bit of drama, so that we might think he'll accidentally poison himself.
I don't think your theory does fall apart (although I don't agree with it). He doesn't directly kill the husband by poisoning. He poisons the wine, but the mother tells the daughter to take it, and the daughter actually gives the husband the glass. It is very indirect. It is true that he never directly kills anyone, somebody else always acts on his behalf.
"oh mummy, oh daddy - lets all play Kabadi!"