First of all I want to say thank you ma for the opportunity to learn God's word.
I will start from the definition of mercy quoting from you as I see it as the best for now as.. Mercy is God compassionately withholding what we rightfully deserve because of our sin.
And now to what would happen to men if God were not merciful, God thought can only be found his word and to what would will happen is found there as well. So what would have happened can be seen in Genesis chapter 6,the flood which is total destruction, secondly is in Genesis chapter 19, the destruction of Sodom and Gomorrah. Thank you.