Guilt and assumptions

Lady Macbeth, contemplating the sleeping King Duncan.
image from Wikipedia

          In act 2 scene 2, Macbeth tells his wife that the two servants had awoken before he could kill them  (lines 30-44). Included in this section of dialogue is more proof that he knows is actions are wrong

"As they had seen me with these hangman's hands,
List'ning their fear. I could not say "Amen"
when they did say "God bless us". (lines 37-40)

On line 44, Macbeth says that the Amen "stuck in his throat". what he did was murder, and he knows it so deep within the core of his being that his voice rebels at this false act of piety. His later inability to sleep is a further expression of this guilt.

      Later, in scenes 3 and 4, we learn that Macbeth's claim that he discovered the servants in the act of murder and killed them, has been accepted, and that Donalbain and Malcolm are suspected of ordering the assassination, since they fled out of fear for their own lives. why is the suspicion immediatly on them? was there any indication of resentment from either?