In both scenes, the character, Frankenstein or Macbeth, is sharing his emotions in very descriptive language.
In chapter 5 of Frankenstein, Victor Frankenstein begins by praising himself for the beautiful creature he has created.
How can I describe my emotions at this catastrophe, or how delineate the wretch whom with such infinite pains and care I had endeavoured to form? (Frankenstein ch 5, p. 29)
Then, Frankenstein sees his creation looking at himself.
I started from my sleep with horror … I beheld the wretch—the miserable monster whom I had created. (Frankenstein ch 5, p. 29)
At first he is very pleased with himself, but then he begins to regret his decision when it fully dawns on him what he has done. He has created a monster by bringing this being to life. He cannot face what he has done, and he runs away. The monster has done nothing to him. He just regrets its existence and deserts it.
In Macbeth, Macbeth also bares his feelings vividly. At the beginning, he is talking about the murder of Ducan, just as Frankenstein described the creation of the monster.
If it were done when ’tis done, then ’twere well
It were done quickly. If the assassination
Could trammel up the consequence, and catch,
With his surcease, success; (Macbeth Act 1, Scene 7, p. 22)
As with Frankenstein, Macbeth has a moment of doubt when he realizes what he is about to do, but he does it anyway.
He's here in double trust:
First, as I am his kinsman and his subject,
Strong both against the deed; then, as his host,
Who should against his murderer shut the door…(Macbeth Act 1, Scene 7, p. 22)
Macbeth does kill Duncan. He does not regret it until later, but like Frankenstein he has a moment of doubt when he hears that he has murdered sleep and will sleep no more.
In each case, the language is very dramatic and colorful at this point. The vivid imagery sucks us in, and we can’t wait to find out what horrors will fall upon each man.