The answer can be summed up in one phrase: overreaching ambition. Both the characters of Frankenstein and Macbeth are defined by their ambition that knows no limits and pushes them to exceed the bounds of sensible reason and break through important barriers of what should and shouldn't be done. Note what Frankenstein remembers his feelings were after starting university and hearing a lecture that suggested science could be used to bestow life on a human form:
So much has been done, exclaimed the soul of Frankenstein--more, far more, will I achieve; treading in the steps already marked, I will pioneer a new way, explore unknown powers, and unfold to the world the deepest mysteries of creation.
Frankenstein clearly here talks of his ambition, his overwhelming desire to go beyond what has already been discovered and to know no limits or boundaries in his scientific exploration. This is of course the massive tragedy of the novel.
With Macbeth, it is again his ambition that causes him to kill Duncan and commit regicide, a crime that was considered to be one of the most terrible in Shakespeare's days. Note what he says about ambition in one of his famous soliloquies from Act I scene 7:
I have no spur
To prick the sides of my intent, but only
Vaulting ambition, which o'erleaps itself
And falls on the other.
Macbeth recognises the danger of giving oneself over to ambition so completely, yet thanks to his wife he is unable to check himself, and advances on his path to perdition.
For both characters, then, the unifying characteristic is that of ambition and how it dominates them both, leading them to tragedy and ruin.