Will Krishna show His Mercy to the most sinful persons? 

Srila Prabhupada speaks:

“In this material world, Krsna has certainly arranged for our care,  but also for our punishment. If you commit this sin, then slap. If you commit  that sin, then kick. 

This is going on under the heading of the threefold miseries--(i) those caused by our own body and mind,  (ii) those caused by other living entities, and  (iii) those caused by natural calamities under the supervision of the demigods. 

Unfortunately, instead of understanding that we are being punished for sinful activities, under the spell of maya, we are thinking that this kicking, slapping, and thrashing are accidental. This is illusion. 

As soon as you take up Krsna consciousness,  Krsna begins personally taking care of you. 

As He promises in the Bhagavad-gita (18.66):  "I will take care of you. I will save you from all sinful reactions. Do not worry." 

Because we have had so many lives in this material world, we are suffering under heaps of sinful reactions.  But as soon as you surrender to Krsna, He immediately takes care of you and nullifies all your sinful reactions. 

Krsna says, "Don't hesitate." Don't think, "Oh, I have committed so many sins. How can Krsna save me?"  

No.   Krsna is all-powerful.   He can save you. Your duty is to surrender to Him and without any reservation dedicate your life to His service. Then Krsna will save you without a doubt. “