He was on a two-week leave to visit family and friends in Houston where he grew up.
Sunday evening, as he was standing outside his father's house in the 5600 block of Newport Drive, he was shot to death.
According to the Houston Police Department, the shooter had an "AR-15 style rifle" and sped off toward the Gulf Freeway.
Castillo's father, 39-year-old Albert Castillo Sr. was also shot in the arm and survived. The airman's family told our sister station ABC13 he was at the wrong place at the wrong time.
"He was so young," said his aunt, Cynthia Ortiz. "He could have done so much more. He could have touched so many more lives. We lost a mentor. We lost a man of God. We lost a son, a nephew, a brother. We lost so much."
Castillo graduated from Eastwood Academy High School but played football for Austin High School. He was taking classes at Houston Community College when, last year, he decided to join the Air Force.
"He was a very brave man. The honor of serving his country was important to him," said Ortiz.
Ortiz said he will be given a military burial with honors, but arrangements have not been finalized. She and her family want his killer caught.
"Not just caught and let go, we want [the shooter] to suffer," said Ortiz. "We want justice for [Castillo]. We need justice for [Castillo]. It's not fair."
Houston police described the shooter as having a tattoo sleeve on his left arm and a thin beard. There is no known motive, according to HPD.
Anyone with information is urged to call the HPD Homicide Division or Crime Stoppers at 713-222-TIPS.
Meanwhile, the family has established a GoFundMe.