This is from Professor Google:
"'Suicide' refers to a drink made by mixing a small amount of every available soda flavor from a fountain into a single cup. While there is no single 'official' origin story, the name is primarily a piece of American childhood and teen slang that emerged between the 1950s and 1970s. The name likely stems from the idea that mixing so many clashing flavors is a risky endeavor that could 'kill' you. It was often treated as a dare or a test of one's stomach.
Depending on the region, the same drink is known by other grim names that imply a similar 'deadly' or 'unpleasant' nature. 'Graveyard' is common in the Pacific Northwest and parts of the East Coast. 'Swamp water,' referring to the murky brownish-green color resulting from the mix, is popular in the Midwest and Canada, Other names include 'sewage,' 'pop bomb' and 'tornado.'"