The law is there to show us how sinful we really are. Naturally, as you pointed out, we should all be put to death. This also shows the true level of Gods mercy: that while we were still sinners, (while we were awaiting execution for all of eternity and rightfully deserved our death penalty), God sent His only Son to die for us in our place. Thank God, that we are now offered forgiveness through His Son if we truly believe, and truly ask for forgiveness out of a sincere and repentant heart. How awesome our Father is!