Two good passages to consider are Hamlet I.ii.132-162, which begins
O, that this too too sullied flesh would melt,
Thaw and resolve itself into a dew,
Or that the Everlasting had not fix'd
His canon 'gainst self-slaughter!
and Macbeth I.vii, beginning
If it were done when ’tis done, then ’twere wellIt were done quickly. If the assassinationCould trammel up the consequence...
even-handed justiceCommends the ingredients of our poison’d chalice
To our own lips.