To kill in battle is one thing and Macbeth is good at that, but to murder a defenseless human being, his king for that matter, is a totally different thing.
To Macbeth, on paper, the plan to kill Duncan is one thing but when he begins to contemplate the reality, he is able to assess the situation. Weigh the pros and cons, so to speak.
He knows that if he kills Duncan, he himself will pay for it eventually with his life. Judgement in the afterlife is one thing to think about but as he says, "We still have judgement here,..."
He thinks of all the reasons not to kill Duncan. He is a kinsman and Duncan is his king. He is his host and that's not the way hosts are suppose to behave. In fact, they should work to prevent such an action. He has been a good king . Heaven itself would condemn whoever killed the king.
By the time his wife joins him, he has decided against killing Duncan. The prophesy and his own ambition are just not enough for him to kill Duncan. It is only when Lady Macbeth goads him by questioning his manhood does he change his mind.
I have assumed that, when you mention Macbeth's soliloquy in Scene 7, you are referring to Act I, scene vii. This is one of the most famous soliloquies in the play. Macbeth has misgivings on a few counts.
First he is concerned that there is no way that the simple act of killing Duncan would ever be just that -- a simple act. He says that if that could be "the be-all and the end all" then he would "jump the life to come," meaning there would be no more reason to consider not killing Duncan. He would "jump" in and do it, no more questions asked.
He next considers how, by committing a "bloody" deed against his fellow, he is merely creating his own "justice" which will surely catch up with him in the end, giving him his own self-induced punsihment, his just desserts.
He also notes that it is added to his karmic debt of "just desserts" that Duncan has come in trust as a guest under his roof, plus he should have the trust of Macbeth, since Macbeth is his subject. Duncan is also a man of great and noble actions, whose legacy will live on after his death, gathering the pity of all.
Macbeth concludes with the observation that the only thing he has to stack up against all these misgivings is the thing that will drive him to his downfall, his tragic flaw -- ambition. For ambition, Macbeth ignores misgivings that he knows are correct, sealing his own fate in the choice.
The links below will provide further insight into this very famous soliloquy.