http://revach.net/article.php?id=4283

Section: Parshas HaShavua   Category: Quick Vort
Parshas Shemos: Vilna Gaon - When Paroh Died Complete Chaos Broke Loose
"Vayamas Melech Mitzrayim Vayei'ancho Bnei Yisroel Min HaAvodah", the King of Mitzrayim died and the Bnei Yisroel groaned from the work (Shemos 2:23).  The Vilna Gaon asked why would the Bnei Yisroel have a bigger problem with the work after Paroh died (See also Rashi)?

The Vilna answered that it is not the ways of a well run Kingdom, no matter how cruel the King is, to persecute people extra judicially without justification.  This could cause a popular uprising by even proponents of the persecution out of fear of a too powerful king. 

It was because of this says the Gra that Paroh instructed Shifra and Puah to kill the male newborns only during delivery and not once they were out.  We see this from their response that they didn't kill the boys because they always came too late and missed the delivery.

As long as Paroh was alive their was a vague semblance of justice, despite the torture as everything needed a justification.  However when Paroh died and a new King had not yet been crowned, it was a free for all with each Mitzri fully taking advantage of this power vacuum to brutally beat his Jewish slave.