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.
Because it is your personal opinion & judgement
It's a metaphor.
To help you understand why, here's the definitions of both:
Simile: it's a <span>figure of speech involving the comparison of one thing with another thing of a different kind, it's used to make writing seem more vivid.
Metaphor: this is also a</span><span> figure of speech in which a word or phrase is applied to an object or action to which it is not literally applicable.
</span><span>
</span>
