1.The molten law is not symbolic. God does not use molten lava to burn people alive for hours or days. Molten lave can purify the earth of the dead however.

2.Regarding the cruelty question, no, God was not cruel.

Quote:
By stubborn rejection of divine love and mercy, the Jews had caused the protection of God to be withdrawn from them ... (GC 35)


If it were God's idea to cause those who rejected Him to be burned (as you are suggesting) that would be cruel. However, God was caused to withdraw, which is another matter.

Those who wait for the Bridegroom's coming are to say to the people, "Behold your God." The last rays of merciful light, the last message of mercy to be given to the world, is a revelation of His character of love.