How can you describe Macbeth's attitude to Duncan—as seen in his parting words?

Macbeth has no parting words, as such, with Duncan, but Macbeth makes other references to Duncan throughout the play that reveal Macbeth's true feelings towards him.
On Macbeth's first meeting with Duncan, Macbeth describes his loyalty to his king:

MACBETH. The service and the loyalty I owe,In doing it, pays itself. Your highness' partIs to receive our duties, and our dutiesAre to your throne and state, children and servants,Which do but what they should, by doing every thingSafe toward your love and honor. (1.4.25–30)

The last words that Macbeth speaks to Duncan in the play occur just a few lines later, after Duncan tells Macbeth that he intends to visit Macbeth at Inverness, Macbeth's castle:

MACBETH. I'll be myself the harbinger, and make joyfulThe hearing of my wife with your approach;So humbly take my leave. (1.4.51–53)

Macbeth is pleased that Duncan is going to visit him at his castle and looks forward to telling Lady Macbeth the good news, but before making his exit Macbeth expresses to himself his dissatisfaction with Duncan naming his son, Malcolm, as his successor, and Macbeth reveals his ambitions towards acquiring Duncan's throne:

MACBETH. (Aside) The Prince of Cumberland! That is a stepOn which I must fall down, or else o'erleap,For in my way it lies. Stars, hide your fires;Let not light see my black and deep desires:The eye wink at the hand; yet let that beWhich the eye fears, when it is done, to see. (1.4.55–60)

Macbeth doesn't meet Duncan when Duncan arrives at Inverness and avoids meeting with him at dinner.
After Macbeth discusses killing Duncan with Lady Macbeth, Macbeth has second thoughts it and thinks about his duties to Duncan and his feelings towards him:

MACBETH: 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. Besides, this DuncanHath borne his faculties so meek, hath beenSo clear in his great office, that his virtuesWill plead like angels trumpet-tongued againstThe deep damnation of his taking-off (1.7.12–20)

We don't know what Macbeth might have said to the sleeping Duncan before he murdered him, but Macbeth expresses nothing but remorse, regret, and revulsion for having killed him:

MACBETH: This is a sorry sight. (2.2.28)

Macbeth heard the guards praying before he killed Duncan:

MACBETH: One cried, "God bless us!" and "Amen" the other,As they had seen me with these hangman's hands.Listening their fear, I could not say "Amen,"When they did say "God bless us!". . . But wherefore could not I pronounce "Amen"?I had most need of blessing, and "Amen"Stuck in my throat. (2.2.36–43)

When Macbeth forgets to leave the daggers with Duncan's guards, Lady Macbeth tells him to take the daggers back to Duncan's room.

MACBETH: I'll go no more:I am afraid to think what I have done;Look on't again I dare not. (2.2.64–66)

There's knocking at the castle gate, and Macbeth clearly wishes that he hadn't killed Duncan, and regrets that he's done so:

MACBETH: (Knocking) Wake Duncan with thy knocking! I would thou couldst! (2.2.93)

He later remarks, "the gracious Duncan have I murder'd" (3.1.70).
The last words that Macbeth speaks about Duncan express his desire that Duncan rest in peace:

MACBETH: Duncan is in his grave;After life's fitful fever he sleeps well;Treason has done his worst: nor steel, nor poison,Malice domestic, foreign levy, nothing,Can touch him further. (3.2. 24–28)