Lyric discussion by TitoP1mp 

Cover art for Killing The Fly lyrics by Union Underground, The

Great song! Looks to me like this song is about marijuana addicts (Mary being Mary Jane) and about someone who was able to break out of the addiction and is now watching other addicts and trying to tell them that they're ruining their lives, but at the same time the reformed person is laughing at the stupidity of being addicted.