If it were done when 'tis done, then 'twere well It were done quickly. If th' assassination Could trammel up the consequence, and catch With his surcease success—that but this blow Might be the be-all and the end-all!—here, But here, upon this bank and shoal of time, We'd jump the life to come.
William Shakespeare (1564–1616), British dramatist, poet. Macbeth, in Macbeth, act 1, sc. 7, l. 1-2.
"If it were done when 'tis done" means if the deed were at an end, completely finished, at the moment it is done; Macbeth thinks that if he could avoid ("trammel up" means catch in a net) the consequences of murder with the death ("surcease") of Duncan, he would take a chance on the life to come (on earth, and in heaven or hell).