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. Prior to the NBA, Hayes began playing professionally in France. In 2017, he won MVP of the U16 Euro Championship A. During the 2017-18 season with Espoirs Cholet and Cholet Basket, the guard averaged 13.5 points, 5.9 assists, 3.4 rebounds and 2.1 steals in 25.6 minutes. He also was selected to the 2018 U17 World Cup All-Tournament Team. The following season, Hayes remained with Cholet Basket and averaged 7.1 points, 3.1 assists and 2.3 rebounds in 19.7 minutes. During his final campaign overseas, Hayes played for Ratiopharm Ulm in Germany. He averaged 11.6 points, 5.4 assists, 2.8 rebounds and 1.5 steals in 24.8 minutes. Arguably Hayes' best performance in 2019-20 was Nov. 17 in a win over Brose Baskets Bamberg, where he recorded 24 points (9-13 FG, 2-3 3Pt, 4-4 FT), five assists, three steals, two rebounds and one block in 30 minutes.