“As two spent swimmers that do cling together/ And choke their art” (I.II.10)
"Look like th' innocent flower/But be the serpent under't" (I.VI.76-78).
There are three witches that plan on meeting with Macbeth. While talking about their plans, they bring up the idea that bad is good and good is bad.
Macbeth is in battle against Macdonwald. During the battle, Macbeth gets the better of Macdonwald and is able to kill him.
This is a motif for fair vs foul and deception. The idea to look and act all innocent and pure like the flower shows the idea of being fair. However, faking people and actually being the snake shows deception. The snake also resembles foulness.