Near the end of Act 1, why does Macbeth express doubt about murdering the king?

Macbeth expresses doubt about murdering the king for several reasons.  He says, "If it were done when 'tis done, then 'twere well / It were done quickly" (1.7.1-2).  In other words, he knows that simply killing Duncan will not automatically make him king.  There will be more to do, lots more, in order for Macbeth to secure the crown; it will be neither easy nor quick.  Further, he says that "Bloody instructions [...] being taught, return / To plague th' inventor.  This even-handed justice / Commends th' ingredience of our poisoned chalice / To our own lips" (1.7.9-12).  Here, Macbeth is concerned that by committing this violent act, he inadvertently teaches others to commit violence as well, and this could come back to bite him later if someone treats him as he treats Duncan. 

Then, more simply, Macbeth is Duncan's "kinsman and his subject" as well as his host, "Who should against his murderer shut the door, / Not bear the knife myself" (1.7.15-16).  It is Macbeth's duty to protect Duncan for so many reasons, not the least of which is that they are related and that Macbeth is currently hosting the king at his own home.  It is hardly hospitable to murder one's guest and one's cousin.  Macbeth goes on to describe Duncan's goodness, the fact that he is such a virtuous man and good king, and these make him doubt his resolution to kill Duncan as well.