>Ok, so as an athiest, could you do what is right, love mercy, and walk humbly with your beliefs?

What has caused you to conclude that any of those things are right? Mercy, benefit of doubt, leniency... these are for the remorseful. We live in a society that doesn't even seem to know that word.

>However, justice also limits the cruelty.

Within reason. Haven't seen much evidence that we currently need to limit it. This isn't 1700s England where anyone is getting the noose for stealing loaves of bread.

Show me some excessive punishment, I might agree with you. "They have to live with the summer heat like everyone had to do so, convicts and the innocent alive, until 70 years ago" just isn't excessive.