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).