Macbeth finds the idea of killing King Duncan to be repugnant for several reasons. As Duncan sleeps in Macbeth's castle, shortly before his murder, Macbeth thinks of all the reasons Duncan should not be slain. There are matters of honor and responsibility. Macbeth says that Duncan is "here in double trust." This means, as he explains, that Macbeth is King Duncan's subject and also his "kinsman," suggesting a family tie. He continues that he is also Duncan's host "[w]ho should against his murderer shut the door, / Not bear the knife myself." In other words, Duncan trusts Macbeth without question and sleeps peacefully, unaware of his impending death.
After examining his responsibility in the matter, Macbeth also speaks of Duncan's many personal virtues and what an excellent king he has been. Killing him will be a "deep damnation," so terrible that "tears shall drown the wind."
Despite these strong feelings against what he is about to do, Macbeth does it anyway, murdering Duncan in his sleep.