I think the main question here is "why Viggo calls John to tell him he killed his friend?", while he could have just tried to discreetly run away far from John.
As mentioned, Viggo is smart and knows how dangerous and implacable John Wick is.
I think deep down Viggo realizes that he is not gonna make it, John will find him anyway wherever Viggo runs away.
As an organization leader, Viggo keeps up appearances by setting up an escape attempt in helicopter, but the only thing that he really hopes for any more, is a final confrontation with his old acquaintance John Wick, to go out with a bang.
In the car driving Viggo and his guys to the helicopter, we can see Viggo laughing with pleasure. During the final confrontation, he also gets opportunities to exchange punching and "tasteful" dialogue: "What happened to us John? We used to be civilized, professional." - John: "Do I look civilized to you?"
So I believe Viggo called John about Marcus because that is what settled the destiny he had chosen for himself, setting up a last fight with John, rather sooner than later.