I don't think Mason intends to kill his dogs. That is simply the impression of the viewer, because of the sinister character and the atmosphere of the scene. Besides, that would only accuse him (his visit would be registered) and they are going to be sacrificed by court order. On the other hand, chocolate is bad, but not lethal for dogs. I doubt that the health argument would have been taken into account (for example, in "Anatomy of a Murder", the dog drinks beer and nobody was shocked).
As I've said before, killing them would only incriminate him. For me, that "one more favor" is to be sacrificated, what would happened in a few days. But the scene of the visit seeks to produce tension in the viewer and succeeds in doing so.