Here is an interesting article that lists a lot of why? questions.
Most of the answers that I think of do not add up.
Like I said in another comment. They are really piling on the bs on this shooting.
http://raconteurreport.blogspot.com/2017/10/who-how-why-and-wtf.html
Other possibilities from murder mystery writers:
To keep a secret - Unlikely
Revenge - No
Frustration/hate - Because of...?
Money/Greed - No
Sex/Jealousy - No
Property Dispute - No
Personal Vendetta - Against...?
Political - No
Class Conflict - No
Narcotics - No
Other Felonies - No
Urge to Protect - No