How does L.Macbeth line, " a little water clears us of this deed. How easy is it the." Contrasts with macbeths line, " what hands are here?.....making the green one red,"

L. Macbeth tells Macbeth this just after the murder of Duncan. The irony is very telling here, considering Macbeth's statement about Neptune's oceans. While Macbeth laments his inability to cleanse his guilt, L. Macbeth points out that water will clear him of his crime. L. Macbeth's comment comes off as weak and naive.