Macbeth's behavior in these scenes, as in the play as a whole, is a gigantic paradox in that he apparently feels genuine loyalty to Duncan while simultaneously he begins to plot not only his death but that of Malcolm as well. From the moment the prophecy is stated by the witches Macbeth acts as if in a trance, propelled by a supernatural force that turns him against his better judgment as well as his moral conscience. A second, more specific paradox can be seen in the interaction that occurs between Macbeth and Lady Macbeth in I.5. He has already been contemplating the steps necessary to fulfill the prophecy; yet without Lady Macbeth's urgings it is entirely possible that he would have dropped the whole matter simply because there is that spark of humanity within him that Lady Macbeth lacks (though she, too, is not completely evil because she is struck by the resemblance between Duncan and her father without which she would presumably have killed him herself). So it is paradoxical that it is Lady Macbeth who drives Macbeth into the vortex of violence that is his (and her) undoing, yet the seeds of the crime are within Macbeth himself--or at least the supernatural power that has taken possession of him.
"nothing in his life / Became him like the leaving it . . ." (1.4)
"murdering ministers" (1.5)
The only help you'll need to explain the passages is a definition of paradox--just type the word into an online dictionary, and you should be good to go.