Mosaic Law represented God's wishes, not a watered-down version to gradually civilize Israel.
A common Christian excuse for the uncivilized or savage Mosaic Laws is that God was utilizing progressive revelation in an attempt to gradually civilize the Israelites. This would mean that God initially delivered a watered-down version of His Law to make it easier for them to comply and then gradually civilize them with progressive alterations to the Law.
This is a false claim for the following reasons:
Scripture says God's Law is perfect. A watered-down version would not be perfect.
God did not expect (anticipate) obedience to the Law. There would be no reason to offer a watered-down version in order to make the Law easier to follow if obedience was never expected. If the Law was meant to demonstrate that humans are imperfect and need a savior, the Law needs to be perfect for this demonstration.
Progressive revelation does not work this way. In order to gradually build upon the basic principles, the basic principles must remain true throughout.
If my goal is to teach my child calculus, I will begin with the basic principle that 1+1=2. I will build upon this basic principle with subtraction, multiplication, division, fractions, percentages, algebra, geometry, trigonometry, and ultimately calculus. All throughout this process, 1 plus 1 will always equal 2 and this basic principle will never change. More complex concepts will be added, but the basic principle remains in effect.
If my goal, as God, is to teach my children to love their neighbors as they love themselves, and I start with the basic principles of "slaughter every living creature in the towns of your enemies, and kill your own neighbor if he works on the sabbath", how do I progressively build upon these basic principles in a way that I reach my goal and the basic principles remain in effect?
ADDITION: The argument for progressive revelation is often utilized by Christians defending Objective Morality.
If Mosaic Law is perfect and was intended to serve as a demonstration of perfection which mankind was unable to achieve (thereby needing a savior), then Mosaic Law must also be in perfect harmony with MORALITY.
Since it would not be possible for God to judge a person as being perfect according to the Law while simultaneously judging the person to be immoral, the Law must equally represent MORALITY.
If OBJECTIVE MORALITY exists, it does not vary according to who you are, where you live, and it doesn’t change over time.
If OBJECTIVE MORALITY exists, it must be embodied in Mosaic Law and it cannot have changed.