Back in the day, when all the kids I knew played soccer instead of those wimp sports like baseball and football, everyone got treated to a soda after the game was over. While some kids insisted on Coke or Pepsi or Sprite or whatever the "cool" soda was, all the really smart kids ordered a Suicide.
A Suicide was what we called a mixed soda with a little bit of several different sodas. Some ordered a "Coke/Orange Suicide" or a "Pepsi/Root Beer/Sprite Suicide", while others ordered a "Suicide with Everything", which mixed together a little bit of every soda at the concession stand.
One assumes that the name came about because some poor schmuck thought that drinking root beer mixed with orange soda was a risky undertaking.