20 Lord is a term of high respect in this context. God’s presence in the work of Jesus makes Peter recognize his authority. This vocative is common in Luke (20 times), but does not yet have its full confessional force.
21 Peter was intimidated that someone who was obviously working with divine backing was in his presence (“Go away from me”). He feared his sinfulness might lead to judgment, but Jesus would show him otherwise.

In other versions

But when Simon Peter saw it, he fell down at Jesus’ knees, saying, “Depart from me, for I am a sinful man, O Lord.”

When Simon Peter saw this, he fell at Jesus’ knees and said, “Go away from me, Lord; I am a sinful man!”

When Simon Peter saw this, he fell at Jesus’s knees and said, “Go away from me, because I’m a sinful man, Lord!”

When Simon Peter realized what had happened, he fell to his knees before Jesus and said, “Oh, Lord, please leave me—I’m such a sinful man.”

And when he saw it, Simon Peter fell down at Jesus’ knees, saying, “Depart from me, Lord, because I am a sinful man!”

When Simon Peter saw it, he fell down at Jesus’ knees, saying, Depart from me; for I am a sinful man, O Lord.

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