There seems to be three views on this.
First, that alcohol is responsible for "clouding" the brain and producing "the monster from hell". The effect of alcohol directly creates Mr. Hyde.
The second view is that the monster is always there but that alcohol allows Mr. (or Ms.) Hyde to escape and temporarily be seen.
Third is the view that Mr. Hyde is always present, but that alcohol makes the monster even worse.
I tend to believe the reality is somewhere between the second and third view (depending on the individual).
"To confront a person with his own shadow is to show him his own light".
C.G.Jung
This is because the shadow is most dangerous when unseen.