Lack, want, need, require as verbs all stress the absence of something desirable, important, or necessary. Lack means to be without or to have less than a desirable quantity of something: to lack courage, sufficient money, enough
Macduff's son is a character that even though, his age is not stated in the play, is assumed to be young. The killing of this young character and especially the way that happened shows Macbeth's kingdom cruelty. The murderers call this boy "Egg" while he tried to defend himself and defend his mother who was later killed too.
The first one is an appeal to the reader's sense of logic and the second is an appeal to the readers emotions
