The days of Malachi were much like the days we are living in today. Totally corrupt; wickedness abounding; days of spiritual indifference. There is a curse at the beginning of the book (2:2)… in the middle (3:9)… and the book ends with the word “curse” (4:6). Darkness had settled in.
Most heresies are Bible doctrines that have been misapplied—Sabbath keeping; losing salvation; speaking in tongues; baptismal regeneration, etc.