The Toll of Murder

In scenes three and four, we see MacBeth turn from a man feeling the immense guilt of one murder, to a maniacal killer, willing to do anything to maintain his power.  I do not understand how this can happen over such a short time.  Within a matter of days, MacBeth has either killed off or alienated all of his friends and turned into a cold-blooded monster.  The guilt of one murder, instead of acitng as an inhibitor, acts as the driving force behind the following massacres by MacBeth.  His wife, many predicted to be the driving force behind MacBeth's ambitious characteristics, is now deathly afraid of the monster she has made.  I do not believe that the obsession that overtakes MacBeth is realistic for the time period set forth by Shakespeare, however, Shakespeare puts forth a fast-forwarded version of what can happen to an ambitious man with no moral restraints.  Guilt and suspicion plague MacBeth's concience as he strives forward to maintain his reign.