I always imagined Anakin slowly turning to the darkside over a period of time spanning many years.
As was the case with the films we have. During the 9 years between TPM and AOTC, things went horribly wrong. Unfortunately we don't get to see the journey from innocent young boy to twisted adult.
In ROTS, he turns from being a pretty typical human being into a sick, twisted, sadistic, madman within a couple of days, perhaps even hours.
False. He was always a sick, twisted, sadistic madman from AOTC-ROTS. Nothing changed. "Killed women and children" and "was blatantly treachorous" all the while, as ConfusedMatthew so superbly put it.
Did it ever occur to him before he tries to kill all those jedi and Padme, he sleeps on it??!!!!
Anger, jealousy, selfishness... all part of the dark side. These emotions in great intensity cloud judgment, and everything we know from the OT seems to only indicate that the dark side augments these emotions.
It would've been a real drag for him to wake up the next day in the vader suit and think to himself; man, what was I thinking yesturday!
Which he most likely did, to an extent. Which would lead to ambivalance: Vader hates himself for his choices, yet turns around and makes excuses for them. Great inner conflict would only drive his evil actions, because he sees that excercising great control over others and his surroundings, he may gain control over his inner conflicts. An ongoing, doomed-to-failure battle.