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Gunter Hermann Ewen
As a youth: theft, fraud and battery
Starting in 1982: caught numerous times while driving under the influence of alcohol, which resulted in a three month prison term when he was 26
Since 1990: was arrested for acts of voyeurism and exhibitionism
1991: was fined after masturbating in front of women at a tennis club. Six months later Ewen, in drunk condition, twice broke into a home for mentally disabled women and sexually molested them in their beds. One hour after the second such incident he raped a woman in her home.
1993: Ewen was convicted for rape and sexual assault in three cases, and sentenced to a prison term of five years and six months.

Devin Patrick Kelley (Sutherland Springs shooting):
2012: Beat his wife and fractured his stepson's skull. Charges of pointing a weapon at his wife were dropped.
2014: Discharged from US air force for bad conduct.

Charles Andrew Williams: (killed 2 and injured 13 at Santana High School in 2001)
11/12 years old: (supposedly) set a large fire in a forest.
12 years old: started using cocaine and other hard drugs.
14 years old: smoked cigarettes, smoked marijuana, drunk underage, robbed a liquor store for alcohol (with friends), stole friend's mum's pain pills constantly, was a repeated truant.
15 years old: was found in possession of alcohol underage, probable hard drug use, repeated truancy, possibly raped/molested a 12-year-old girl.

Itamar Alon (2013 Beersheba shooting, 4 killed and 5 injured, suicide)
In 2011, Itamar Alon, damaged his neighbor's air conditioner and then barricaded himself in his apartment. The Isreal Police wanted to take away his gun, but he was allowed to keep the gun that he would later use in the shooting.
Akiyoshi Umekawa (1979 Mitsubishi Bank sporting gun kidnapping, 4 killed, killed by police, being the last criminal killed by japanese police, until 2018)
At age 15, he killed a woman. Due to his young age, he had his criminal record protected which allowed him to get a gun.