In chapter 1 of the book of Malachi, the Lord speaks through this prophet to the people of Israel saying that he has clearly loved them, but that they do not honor him due to their faults, the people were doing things as they wanted and not as God wanted. That is why the Lord reminds them of the love He has for them.
This entire chapter speaks to the people as well as a rebuke to the priests because the Lord speaks clearly to them, and verse 6 says:
A son honoureth his father, and a servant his master: if then I be a father, where is mine honour? and if I be a master, where is my fear? saith the Lord of hosts unto you, O priests, that despise my name. And ye say, Wherein have we despised thy name?
Malachi 1:6
It was clear that the priests themselves were not making the sacrifice that the Lord deserved. That is why the Lord demanded a pure sacrifice before Him.
But cursed be the deceiver, which hath in his flock a male, and voweth, and sacrificeth unto the Lord a corrupt thing: for I am a great King, saith the Lord of hosts, and my name is dreadful among the heathen.
Malachi 1:14
The Lord tells the people that having what is good and what to sacrifice for Him, then they sacrificed what was bad, what was damaged. God was not pleased at all with this evil sacrifice. The people thought they were cheating God by doing this, but they forgot that God knows everything, even our thoughts.
In the same way, let us do things with excellence for God, and even for others, because he who deceives, deceives himself because God cannot be mocked. Everything you sow, that you will reap.