From the lecture..
2016-10-21_SB-10-86-55-59_-_Feeling_Need_to_Cultivate_Relationship_with_Devotees_-_Nanda_Dulal_Prabhu_ISKCON_Chowpatty

What is Krsna's mercy?
When we think of mercy we may say mercy means when you spare a person some punishment that he deserves. If some person has committed a crime, a mistake and he deserves that punishment. By the rule books he deserves to be punished and somebody spares him, somebody pardons him. That is mercy. But when we read about Krsna's mercy then we fall back.

What is Krsna's mercy?
Krsna's mercy is that not that he spared a person who committed a crime but Krsna's mercy is that he gave the position of an assistant to Yashoda Mayi to that person who had come to kill Him when He was the most helpless child.

Those were the days when Krsna did not have the time to turn His back. He was a helpless child. He could not even retaliate even in the smallest way. So , in the most helpless condition when He could not even turn, when He could not even sit. What about crawling and walking and fighting and everything else that He did later with the other demons. He is completely helpless and she comes in the most hideous form. She comes in the most cheating form. She comes as a mother. Mother is somebody who gives life and she came to take away His life. Here Krsna is most helpless and she is trying to kill Him in a most abominable way in a form which is actually full of hope. You look forward to your mother for life, for nourishment and she is trying to do that. And here is the biggest contrast that somebody is very helpless and she is coming in the most evil form ( if we could put it that way).

And what did Krsna give to her?
Yes, He dealt with the situation. Dealing with the situation is very important but taking that consideration that she had that desire to feed Me milk, He gave her the position of a mother. So, this is mercy now. This is your new benchmark of mercy. So, this is how we will define mercy now. Somebody who is trying to attack you, somebody who is trying to kill you in the most unexpected way and you give him the highest reward seeing the good in that person. This is the new upgraded version of mercy now.