Scott Dekraai, 42, is accused of killing eight people at Salon Meritage in Seal Beach last October.
Hair stylist Gordon Gallego told the grand jury Dekraai walked in and told his ex-wife, "this is what you wanted, or this is how you wanted it," and then fatally shot her. The stylist said he also heard a coworker beg for her life before Dekraai killed her. Gallego grabbed another colleague and ran into the employee bathroom, where he heard more than 30 shots and people screaming.
Authorities say Dekraai sprayed Salon Meritage with bullets after he argued with his ex-wife Michelle Fournier earlier in the day over custody of their 8-year-old son. Fournier, a stylist at the salon, and seven others were killed and another person was seriously wounded.
Dekraai is facing the death penalty. He's due back in court August 10.
The Associated Press contributed to this report.