In Srimad Bhagavatam 3.2.23, Uddhava cannot help but praise the Lord's mercy as He displayed to Putana, the terrible demoness who attempted to kill baby Krishna by inducing Him to suckle her poison filled breasts.

aho bakee yam stana-kaala-kootam
jighaamsayaapaayayad apy asaadhvee
lebhe gatim dhaatry-ucitaam tato ’nyam
kam vaa dayaalum sharanam vrajema

"Alas, how shall I take shelter of one more merciful than He who granted the position of mother to a she-demon [Putanaa] although she was unfaithful and she prepared deadly poison to be sucked from her breast?"

Srila Prabhupada puts forward two instructions on how we should learn to act mercifully in this world.

1. "It is said that a noble man accepts the good qualities of a person of doubtful character, just as one accepts nectar from a stock of poison".

A person may have many many flaws in his character, but the devotee extracts his good qualities only, and ignores his negative qualities; in so doing he encourages and enthuses the person of doubtful character to be more inspired to take to Krishna consciousness. In turn, Krishna purifes the person and his flaws eventually vanish.

2. "The Lord accepts the least qualification of the living entity and awards him the highest reward. That is the standard of His character. Therefore who but the Lord can be the ultimate shelter?"

Krishna's merciful character is really revealed here. It is stated that Putana in her previous life was the sister of Bali Maharaj, and when she saw the form of the Lord as a small, handsome, dwarf boy approaching the fire sacrifice of her brother Bali Maharaj, she desired in her heart that she be given an opportunity to mother the Lord someday. The Lord is so kind and merciful that He remembered her desire, and in her next life as Putana, allowed her that great benediction of motherhood, even though she was motivated to kill Krishna. Krishna ignored all the demonic flaws of Putana but only took notice of two things - her previous desire to mother Him, and, as stated by Srila Prabhupada in his purport, the fact that Putana just pretended to behave and act the part of an affectionate mother to get close to baby Krishna without suspicion and eventually kill Him. Even by pretending to like Krishna, Krishna is pleased with the living entity! There is no limit to His mercy! The result of His mercy on Putana was that by killing her, Krishna elevated her to the position of nurse - "dhaatry" - in His eternal pastimes!

We can learn from the above that mercy of the Lord is revealed by ignoring the flaws of a living entity and exalting his noble qualities. Mercy of the Lord also descends when somehow or other, we at least express some sincere desire to serve Him, even if at present we are unable to.