Maybe it's the point, but it's funny how Eric lectures his clients about being in control, but he thinks has to murder his best friend, and maybe his wife. Even if it is because of their affair, that isn't exactly being in control.
Since he's a psychiatrist, he needs to "own those feelings" instead of murdering people.

Hi.

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).

Regards.

Of course he intends to kill them. Mason says, "I'd like to wish you two a long and healthy life, but that wouldn't be so good for me. So, you're going to have to do me one more favor."

What else could he have meant by saying this.

Hi again.

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.

Regards.

While I also thought he meant to kill them, in the italian version he also says he brought them their favourite chocolate: how can it be their favourite chocolate if they never ate it?

Generally I find it strange too that he asks them one more favour if he doesn't mean they should die for him, but the other person said chocolate is not lethal, and combined with it being their "favourite chocolate" I'm not fully sure any more he meant to kill them with it.