The Oklahomla City bombers. The Lewiston Maine shooter. Both Fort Hood shooters.
Link
National Insutitue of Justice
study U.S. mass shootings from 1966 to 2019. "And 28.5% had a military background"
Here is statement before congress about terrorist groups and veteran recruitment.
https://www.congress.gov/117/meeting/house/113968/witnesses/HHRG-117-VR00-Wstate-JonesS-20211013.pdf
In additional is Israel Keyes, many serial killers were in the military. Berkowitz, Ridgeway, Dennis Rader, Rany Kraft, so many others.