There are many reasons why Macbeth hesitates to kill Duncan in Act 1 Scene 7 of William Shakespeare's Macbeth. The first is that murder is not only immoral, in a generic sense, but also is a sin, and Macbeth, brought up in a kingdom that was fully Christianized, would have considered this as condemning himself to eternal damnation. Although at this point in the play, he has begun his downward spiral, it is not until after he kills Duncan that he is fully committed to the path the witches have foretold for him.
The next reason he is reluctant to kill Duncan is that they are kinsmen, cousins to be precise, and killing a kinsman was considered far worse than killing someone who was not a blood relative in this period. As well as being Macbeth's kinsman, Duncan is his king, and so killing him would be treason. Another reason is that Duncan has been very kind to Macbeth, appointing him Thane of Cawdor, and in fact, Macbeth personally likes Duncan and considers him kind and a good king.
Finally, Macbeth is a host to Duncan, and in this period hospitality bore with it strong reciprocal debts of gratitude and responsibility. Someone violating these conventions of hospitality would be considered not only unethical but impious.
Macbeth explains his reasons for his reluctance as follows:
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 ...