Let's look at the verse 9.30:
अपि चेत्सुदुराचारो भजते मामनन्यभाक् ।
साधुरेव स मन्तव्य: सम्यग्व्यवसितो हि स: ॥ ३० ॥
Even if one commits the most abominable action, if he is engaged in
devotional service he is to be considered saintly because he is
properly situated in his determination.
Through the words अपि चेत्सुदुराचारो and साधुरेव स मन्तव्य: Krishna cautions against judging a devotee's accidental falldowns due to his inability to give them up initially during sadhana.
Commentary by Swami Prabhupada: They are a warning to the nondevotees
that because of an accidental falldown a devotee should not be
derided; he should still be considered saintly even if he has
accidentally fallen down. And the word mantavyaḥ is still more
emphatic. If one does not follow this rule, and derides a devotee for
his accidental falldown, then one is disobeying the order of the
Supreme Lord. The only qualification of a devotee is to be
unflinchingly and exclusively engaged in devotional service.
In the Nṛsimha Purāṇa the following statement is given:
bhagavati ca harāv ananya-cetā bhṛśa-malino ’pi virājate manuṣyaḥ na
hi śaśa-kaluṣa-cchabiḥ kadācit timira-parābhavatām upaiti candraḥ
The meaning is that even if one fully engaged in the devotional
service of the Lord is sometimes found engaged in abominable
activities, these activities should be considered to be like the spots
that resemble the mark of a rabbit on the moon. Such spots do not
become an impediment to the diffusion of moonlight. Similarly, the
accidental falldown of a devotee from the path of saintly character
does not make him abominable.
Commentary by Sri Vishvanatha Chakravarti Thakur:
My attachment to my devotee is my very nature. That attachment does
not decrease even if the devotee commits wrong, for I make him come up
to the highest standard. If someone with bad conduct, addicted to
violence, thievery, or adultery (suduracarah), worships me, and
worships no one except me, and does not follow any other process like
karma or jnana, and has no other desire than my desire (ananya bhak),
he is my devotee (sadhuh).
“But, considering his bad conduct, how is he a devotee?”
“He is to be respected (mantavyah) as a devotee because of his devotee
qualities. It is a command. Not doing so is offense. My order is the
authority.”
“So he should be considered a devotee in that portion where he worship
You, and as a non-devotee in that portion where he commits adultery?”
“No, he should be considered as a devotee (eva) in all his parts. You
should not see his bad qualities at all. He is completely convinced
(samyak vyavasitah). He makes a splendid resolution: ‘I will go to
hell for my sinful actions which are hard to give up, but I will not
give up dedicated worship of Krishna.'”
Let's look at the subsequent verse 9.31 to understand this further.
क्षिप्रं भवति धर्मात्मा शश्वच्छान्तिं निगच्छति । कौन्तेय प्रतिजानीहि न
मे भक्त: प्रणश्यति ॥ ३१ ॥
He quickly becomes righteous and attains lasting peace. O son of
Kuntī, declare it boldly that My devotee never perishes.
Krishna uses the words क्षिप्रं भवति धर्मात्मा which mean that the Bhakta will become a Dharmatma or an elevated personality very quickly. He wouldn't have said so if he were a deliberate offender.
Commentary by Sri Vishvanatha Chakravarti Thakur:
"How can you accept the worship of such a sinful person? How can you
eat the food and drink offered by a heart contaminated with lust and
anger?”
“Very quickly he becomes righteous.” The present is used and not the
future to express the fact that having committed sin, by remembering
me, he becomes repentant, and very quickly becomes righteous. “O how
unfortunate I am! There is no one as low as I, bringing bad name to
the devotees.” Repeatedly (sasvat), he feels completely (ni) disgust
(santim).
In short, the Lord is equal to all in granting justice but shows special favor to those who after dedicating their lives in His service accidentally commit some sin and repeatedly repent in their hearts.
Link for commentaries on these verses by various Acharyas to understand them further.
