These words introduce a contrast between Levi and the priests of Malachi’s day, and between their respective service.
In Malachi 2:6 we learn that Levi “walked with God in peace and uprightness.” But the priests of Malachi’s day had turned aside from the way. That is, they turned aside from the path that God had commanded them to walk. They did not follow God’s laws.
Furthermore, in 2:6 we learn that true instruction was in Levi’s mouth, and he turned many people from iniquity, exposing their sin, calling them to repentance, and showing them the path of righteousness. But true instruction was not found in the mouth of the priests of Malachi’s day. Instead, they had perverted the truth, thereby misleading the people, and causing many to stumble by their erroneous instruction, and by the bad example they showed in their own lives.
In 2:6 the implication is that Levi was faithful in the covenant of the priesthood, but the priests of Malachi’s day were unfaithful in that covenant.
8 But you have turned aside from the way. You have caused many to stumble by your instruction. You have corrupted the covenant of Levi, says the LORD of hosts,