Why did God pour out mercy on the whole human race at the cross instead of destroying them?
Because He is sovereign and acts in sovereign grace. At Calvary, God could have destroyed the whole human race, but He withdrew into His sovereignty and poured out mercy—not only for Israel but for the whole human race—because as the sovereign God He could do whatever He wanted.
Les explains that when Israel demanded Christ's death and Rome drove the spikes, God could have cleansed the earth of the whole human race. Instead, He withdrew into His sovereignty. As the sovereign God of the universe, He could do whatever He wanted, and as Christ hung on the cross, He poured out mercy—not just for Israel but for the whole human race—only because He is sovereign. In any other circumstance He would have had to destroy the human race, Israel, and each of us. Even today, what keeps God from totally doing what He was going to do to Israel is His sovereign grace. He is sovereign and in grace, and He poured out His mercy at Calvary.