Zuzak: (is that a name by the way, nickname, meaning?)
One things we must understand when discussing God's justice is our state of depravity. Yes, the Canaanites were particularly wicked, but sin is not a matter of degrees, it is a state of being. All the world is stained in sin, and according to the Bible, are under the wrath of God.
God would be just and good and holy and pure in sending every single one of us to the gallows, or worse. We have sinned, there is not one good, not even one. A good judge leaves no bad deed unpunished or unpayed for. God would be wicked should he allow travesty and debauchery and sin to run rampant across the world unpaid for.
The mercy Jesus proclaims is from His provision. He has paid for the sins of the Canaanites, the rapists, the prostitutes, the businessmen, the phpdip players, you and me. He has done this for free, you nor I have earned this, and he offers this sacrifice to any that would embrace Him and this news as good news.
The Jesus of the New Testament will again return and will crush women and children and people that think they are good and people that think they are not, because they are still under the wrath of God. They are still stuck in sin. They have not embraced His offer of forgiveness, which was himself. My family is still on this side of the equation and it breaks my heart to even write of it. God's wrath is a terrible thing, but it is just.