The book of Malachi is about people and priests with indifference, unbelief, and immorality. This passage in particular is directed to priests; possibly they were not properly cleaning the animals before they sacrificed them. It is also one of many passages that indicate the judeo-christian god is a fecalphiliac.
For a funny video watch this:
http://www.youtube.com/watch?v=1p6mArRqy2c