It's a relationship rarely reflected on in our current culture, but for much of history the role of guest and host were of extreme importance. The crime of MacBeth and his Lady was so momentous that it drove them mad not just because it was murder, and not just because it happened to be their king, but also because it was done while that king was a guest in their house. As ole' Mac tells us himself before doing the deed, "He’s here in double trust: First, as I am his kinsman and his subject, Strong both against the deed; then, as his host, Who should against his murderer shut the door, Not bear the knife myself."