Why did God change his minds about who could cast stones at sinners?

Question

In the OT a sinner was fair game

In the NT only those without sin could cast the first stone

Why did it change?

Answer 1

As Lewis Black once said "I guess he calmed down after havin the kid".

Answer 2

Jesus spoke those words when people brought Mary Magdalen before him; she had been taken in adultery and the elders particularly wished to know if he would have her stoned, according to what is sometimes called the old dispensation. He says, "LET HIM WHO IS WITHOUT SIN CAST THE FIRST STONE". Nobody does because, as Jesus knew, nobody is without sin. But once the crowd disappears, Jesus says to Mary (and this is sometimes forgotten), "Sin no more".
The change from the Old to the New Testament is a complex matter. In Genesis, sin came into the world through Adam and Eve, who disobeyed God's commandment that they not eat of the Tree of Knowledge. Once they did and the Fall came about, the Law, distilled in the Ten Commandments, became necessary for men and women, one of them having to do with not committing adutery, for which stoning was decreed. Jesus declared that the only two commandments which matter have to do with the worship of the one God and loving your neighbor as yourself. Once the emphasis is shifted from the Law to Love, punishment for sin becomes less important than creating a climate in which because of the love people actively feel for one another, the desire to sin becomes less powerful, even if it does not vanish altogether. The power of love blunts the power sin would otherwise have over one.
Incidentally, Jesus did not mean that people without sin could and would cast the first stone. He himself would not. He is much more interested in the reformation of the sinner than in his punishment.
And it should be pointed out that Jesus spoke of himself as extending and completing the Old Testament through his new and revolutionary teaching.
Paul saw Jesus as lifting the burden of sin Adam had brought into the world, through his teaching and his sacrifice: his acceptance of the Crucifixion was the culmination of his love for humanity.

Answer 3

Under the old covenant system, the devil had the power of death, due to sin, and all naturally born men were sinners, under the law.

Under God's new covenant system of grace, Jesus, who is the only one without sin, is the sole judge.