SB 11.21.16

kvacid guṇo ’pi doṣaḥ syād
 doṣo ’pi vidhinā guṇaḥ
guṇa-doṣārtha-niyamas
 tad-bhidām eva bādhate
Word for word: 
kvacit — sometimes; guṇaḥ — piety; api — even; doṣaḥ — sin; syāt — becomes; doṣaḥ — sin; api — also; vidhinā — on the strength of Vedic injunction; guṇaḥ — piety; guṇa-doṣa — to piety and sin; artha — regarding; niyamaḥ — restrictive regulation; tat — of them; bhidām — the distinction; eva — actually; bādhate — undoes.
Translation: 
Sometimes piety becomes sin, and sometimes what is ordinarily sin becomes piety on the strength of Vedic injunctions. Such special rules in effect eradicate the clear distinction between piety and sin.
Purport: 

The Lord clearly explains here that material piety and sin are always relative considerations. For example, if a neighbor’s house is on fire and one chops a hole in the roof so that the trapped family may escape, one is considered to be a pious hero because of the dangerous condition. In normal conditions, however, if one chops a hole in his neighbor’s roof or breaks the neighbor’s windows, he is considered a criminal. Similarly, one who abandons one’s wife and children is certainly irresponsible and thoughtless. If one takes sannyāsa, however, and remains fixed on a higher spiritual platform, he is considered to be a most saintly person. Piety and sin therefore depend upon particular circumstances and are at times difficult to distinguish.

According to Śrīla Madhvācārya, persons above the age of fourteen are considered capable of distinguishing between good and bad and are thus responsible for their pious and sinful activities. Animals, on the other hand, being merged in ignorance, cannot be blamed for their offenses or praised for their so-called good qualities, which all arise ultimately from ignorance. Human beings who act like animals, with the idea that one should not feel any guilt but should do whatever one likes, will certainly take birth as animals absorbed in ignorance. And there are other foolish people who, observing the relativity of material piety and sin, conclude that there is no absolute good. It should be understood, however, that Kṛṣṇa consciousness is absolutely good because it involves complete obedience to the Absolute Truth, the Supreme Personality of Godhead, whose goodness is eternal and absolute. Those who are inclined to study material piety and sin ultimately experience frustration due to the relativity and variability of the subject matter. One should therefore come to the transcendental platform of Kṛṣṇa consciousness, which is valid and perfect in all circumstances.