According to Krishna, what constitutes sin and what is evil?
I believe your question relates to the dialogue between Lord Krishna and Arjuna on the Kurukshetra battlefield just before the start of the great war between Kauravas and Pandavas.
Whether or not these events actually took place has not been established authoritatively. However, the advice and teachings of Lord Krishna to Arjuna, as recorded in the ancient Sanskrit text Bhagwad Gita, is still very relevant and useful today to help people lead a happy life.
Before understanding concept of sin and evil, we need to clarify some other concepts. Lord Krishna considers that the intention and feelings that motivate behaviour of individuals is more important for their long term happiness. The appropriate motivations aim at good of the total society, rather than personal benefits and comforts of the individual. Also, Lord Krishna says that decisions on actions to be taken must be based on codes of conducts, called dharma, designed to promote the welfare of total society in the long run. This has a very significant implication - all action must conform to dharma, irrespective of possibility of failure. This advice is supported by modern concepts of decision making process, wherein decisions are taken on the basis of expected benefits over a long term rather than immediate prospects of loss or gain. John Argenti, a reputed corporate planning expert puts the same idea in these words:
"A best decision does not always give best result, and a poor decision may some ties give very good result."
Once this basic concept of dharma is clear, we can describe sin - called papa in Sanskrit- as all actions not in line with dharma. Evil covers all actions intended to cause harm to others and society.
At times, society may be harmed by some action sanctioned by dharma because of the uncertainty of result. Such action is not sin. But if the intention is to deliberately cause harm to others, rather than total good of the society, then such action is evil. All evil actions are also sins, because they violate the basic requirement of dharma that our action must be guided by good of the total society rather than by considerations of personal benefits.