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.
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.