Macbeth utters these words in Act 1, Scene 7, and we can see that he is in doubt whether he should kill king Duncan. He says:
But in these cases
We still have judgment here; that we but teach
Bloody instructions, which, being taught, return
To plague the inventor...
Macbeth believes that if he resorts to murdering Duncan, there will be some earthly punishments waiting for him. Namely, if one commits some atrocity, one will inspire others to do the same. Killing Duncan may very well backfire on Macbeth once he becomes the king. Murdering someone powerful like Duncan may incite others to harbor similar ambitions for themselves which could motivate them to do the same to Macbeth once he becomes the powerful leader. This is why he says that "bloody instructions, which, being taught, return to plague the inventor."
Macbeth has every reason not to kill Duncan. First of all, he is Duncan's relative and most loyal subject, therefore Duncan always puts his trust in him. Second of all, Macbeth is his host, and he should do anything to protect Duncan, not murder him. In addition, Duncan is a very humble leader, so killing him would be an immeasurable sin.
Nevertheless, he murders him in spite of his doubts, and that moment marks the beginning of Macbeth's downfall.