Question **SPOILER**.
Why Viggo told him the location of his son? He could have made up anything. Told a lie instead.
He then could have left with his son in the helicopter, wherever he was going at the end of the movie. If leaving in the helicopter was not a solution to John Wick problem to him, then if you think about it, the whole ending doesn't make sense, because it was shown as if his leaving in the helicopter would save him from John Wick.
If he thought that John Wick will find him anywhere and he can't hide from him, then why kill Marcus? If he thought that killing Marcus would not entice John to kill him, then why he started to leaving in the helicopter as soon as John sees Marcus dead?
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