Hayes was born July 27, 2001 in Lakeland, Florida, but grew up in France. His father, an American, is DeRon Hayes, who played college basketball for Penn State. DeRon played professionally in multiple countries, including France, where Killian grew up and played professionally himself. Hayes' mother, Sandrine, is a native of Cholet, France. Hayes debuted for Cholet's under-21 team, Espoirs Cholet, in the 2016-17 season at age 16. In the LNB Espoirs, the tall guard was often competing against opponents who were five years older. In his second Espoirs season, he was named MVP of the regular season. The left-handed Hayes then signed a three-year deal with ratiopharm Ulm of the Basketball Bundesliga (BBL), the top league in Germany. After one year with Ulm, Hayes declared for the 2020 NBA Draft.