Kurad is a slur used to describe Estonians in a derogatory way. Originally "Kurad" is an estonian word, which means "Old Nick". Usually it's used by Russians, who are referring to Estonians.
The word Kurad is a racial slur that has been used historically to degrade and dehumanize estonians. The word is still used by some people for this purpose, and it is seen as offensive and hurtful to estonians. If you are unsure about whether or not it is appropriate to use the word Kurad, it is best to err on the side of caution and avoid using it in any situation.