1. "keen" here means both "sharp" and "eager."
2. Lady Macbeth has every intention of killing Duncan herself. Under her "battlements" she will carry out her "fell purpose" with her own "keen knife." Of course, as we see in 2.2., Lady Macbeth fails to commit the murder because Duncan resembled her father "as he slept" (12). The actual reason for her hesitation was likely part true compassion, part simple cowardice.
How to cite this article:
Mabillard, Amanda. Macbeth Glossary. Shakespeare Online. 20 Aug. 2000. (date when you accessed the information) < http://www.shakespeare-online.com/plays/macbeth/macbethglossary/macbeth1_1/macbethglos_keenknife.html >.