“When they finish a game of base ball in Dubuque, Iowa, they toss up a nickle [sic] to see which side shall kill the umpire.” St. Joseph (MO) Daily Gazette, August 20, 1872, p2
Previous earliest use (Dickson Baseball Dictionary, 3rd edition, 2009):
1ST USE. 1876. The Cincinnati Star (June 23; Peter Morris) remarked that the fans who yelled “Kill him” at the umpire were guilty of bad taste.