For all we know after they were "slaughtered" they were visited with the most pure and limitless love, joy, and fulfillment imaginable.
Darn, I had promised not to, but I can't resist... this is just too much :-)
The concept of "heaven" had not been invented at that time (neither the conception of hades in Luke that was probably "borrowed" from the Greek), so all these people could expect after death at that time was "sheol", a place where the dead were said to be "remote from the light of God" (whatever that means, but surely it doesn't mean "love and joy"), regardless of whether they had been good or bad.
As such, there is really only one possible interpretation on how God must have perceived killing those people, since obviously (God knows everything!) he knew that they'd go to a nasty, dark place. And, he knew they were innocent (like a three year old with a burning glass and an anthill).
This conception of afterlife is consistent with "an eye for an eye", too (which much unlike stated in some earlier post can not possibly be interpreted as "lend a blind man an eye". The texts are very clear about the meaning. No matter whether you look in Leviticus, Exodus, or Deuteronomy, the context is always "show no mercy, hack, slash, burn, cripple, kill, kill, kill!").
One might wonder why such harsh, barbaric punishment is openly advocated, but if one considers that dying does not mean "join your god in joy" but rather "go to some dark, desolate place", then it becomes explainable why an offender was expected to face no less but the same fate.