Although I disagree that this is Macbeth's "opening soliloquy," I know the precise moment to which you are referring: Act 1, Scene 7. (It's one of my favorite pieces of evidence, actually, that it's Lady Macbeth and not Macbeth himself that is the true evil force behind this foul murder.) In short, Macbeth gives four precise reasons for not wanting to kill Duncan, ... or "why it would be wrong" as you say.
First, Macbeth is afraid of eternal fire. Macbeth says that "We'd jump the life to come" and they would "plague the inventor." Well, yes, I would say. No one ever wants to "plague the inventor"! Murder is certainly a most foul way to do it, too! It's interesting, in my opinion, that this is the first reason Macbeth gives. It would certainly be enough for me to stop dead in my tracks!
Second, Macbeth is afraid of being killed himself when he does become king. (How ironic!) Look at this line:
This even-handed justice / Commends th' ingredients of our poisoned chalice / To our own lips.
Macbeth, then, fears drinking from this same "poisoned chalice." It's just a fancy way of saying that fate will, most likely, come around. Death will be waiting for Macbeth if he does this, king or not.
Third, Macbeth will be nixing something very important: loyalty.
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.
Macbeth speaks of Duncan here. Duncan is trusting Macbeth in two very important ways. First, as kinsman/subject, Macbeth should always serve the king by protecting him. Secondly, as host, Macbeth should do the same.
Fourth, Duncan is a good king; therefore, he shouldn't be killed.
Besides, this Duncan / Hath borne his faculties so meek, hath been / So clear in his great office, / that his virtues / Will plead like angels trumpet-tongued against/ The deep damnation of his taking-off.
It's pretty amazing, actually, how Macbeth enumerates the reasons why Duncan is a good king. He's humble, "clear in office," and virtuous. By the end of this soliloquy, one wonders why Macbeth is even listening to his foul wife! I'll admit, too, that each time I read it, I root for Macbeth to reveal the plot to Duncan and divorce his wife. I'm afraid I'm always disappointed.