Here are a few red flags from the past 30 mass shooters.
https://www.metro.us/news/map-timeline-of-mass-shootings-in-the-us-since-2000/tmWmll---14WH11UWCww1. Jeffrey Weise killed 10
--multiple suicide attempts
2. Michael McDermott killed 7
--slashed coworker's leg with a knife, rage disorder
3. San Marco killed 7
--talked to herself and walked naked in public
4. Charles Roberts killed 5
--pedophile
5. Sulejman Talovic killed 5
--juvenile delinquent
6. Seung-hi Sho killed 32
--hospitalized for suicidal ideation
7. Tyler Peterson killed 6
--no red flags
8. Robert Hawkins killed 8
--multiple psychiatric hospitalizations
9. Charles Thorton killed 5
--prior assault convictions
10. Steven Kazmierczak killed 5
--schizophrenia, suicide attempt
11. Wesley Hinson killed 5
--history of death threats
12. Jiverly Antares killed 14
--criminal record
13. Oman Thorton killed 8
--no red flags
14. Eduardo Sencion killed 4
--paranoid schizophrenia
15. James Holmes killed 12
--dysphoric mania, homicidal ideation
16. Wade Michael killed 4
--white supremacist militia, reckless alcoholic
17. Adam Lanza killed 27
--Asperger's syndrome, prior death threats
18. Kurt Meyers killed 4
--drunk driver
19. Aaron Alexis killed 12
--history of malicious mischief and disorderly conduct
20. John Zawahri killed 5
--death threats and bomb building
21. Pedro Vargas killed 6
--abused steroids
22. Don Spirit killed 7
--wife beater
23. Dylan Roof killed 9
--drug addict
24. Christopher Mercer killed 9
--asperger's syndrome, stockpiled weapons, suicide attempt
25. Robert Dear killed 3
--weapons violations
26. Syed Farook killed 14
Islamic Jihadist migrant
27. Oman Mateen killed 49
--placed on terrorist watch list for bragging about links to al-Qaeda
28. Stephen Paddock killed 50
--mystery
29. Devin Kelly killed 26
--animal cruelty
30. Nicolas Cruz killed 17
--expelled from school for fighting and online threats