Both share culpability, but Macbeth is the most responsible, for it is he who murders Duncan. However, Lady Macbeth does everything in her power to make it happen. Much like Eve with the apple, events may not have occurred had Lady Macbeth not taken such sure steps to assure Duncan's death, but it is ultimately Adam/Macbeth who puts the apple to his own lips.
As to Lady Macbeth's guilt, when Duncan arrives, she builds up her husband's courage to carry through with the plan to kill him. She instructs Macbeth to beguile Duncan with charm: "...bear welcome in your eye, / Your hand, your tongue: look like the innocent flower, / But be the serpent under't" (1.5.60-64). She warns Macbeth not to wear a troubled expression as doing so might alert their prey: "Only look up clear; / To alter favour ever is to fear: / Leave all the rest to me" (1.5.69-71).
Encouraged by his wife's manipulation and collusion, Macbeth makes up his mind to carry through with the killing. By arranged signal, the striking of a bell by Lady Macbeth announces that the time has come. Macbeth ends his dagger soliloquy by saying to himself: "I go, ...; the bell invites me. / Hear it not, Duncan; for it is a knell / That summons thee to heaven or to hell" (2.1.62-64).
After which Macbeth ambushes Duncan and stabs him to death. Later, Lady Macbeth will realize her guilt. She imagines her hands covered with blood that cannot be washed away.