In this soliloquy, Macbeth reviews the many reasons he has not to move forward with the plan to kill Duncan as well as the one reason that he has to actually go through with the murder.
First, he considers that simply committing the murder will not be enough to make him the king: in other words, it will not be "done quickly." He considers the fact that if he could simply commit this one "blow" and achieve his destiny without consequence, it would be a lot easier to go on and risk his eternal soul.
Next, Macbeth considers the fact that, here on earth, there are punishments, consequences for the type of behavior he's considering. For one, when a person commits a violent act, he inadvertently teaches others how to commit violence, and this can come back to harm the initial perpetrator (i.e. him). Though this would be justice, it could spell death for him.
He considers the fact that Duncan is both his relative and his king as well as his guest, and all these relationships should mean that Macbeth actually protects Duncan from violence, not commits the violence himself. Further, Duncan is such a humble, virtuous leader that his legacy would live on and on, and his death will be such a great sorrow to everyone in the kingdom.
Finally, Macbeth claims that he can think of no reason or motivation to kill Duncan aside from his own "Vaulting ambition" which seems to compel him to rush ahead and, perhaps, rush right into danger.