There's one commonality that seems to be more prevalent in recent years. These murderers tend to write out their intentions and/or manifestos. Agreed with all of the social media blather, it's difficult to differentiate a depressed kid looking for attention or wanting to shock, as opposed to a person who is actually planning a shooting. I mean ffs, it seems that every other teen kid these days is talking about how hard life is and not wanting to continue.
I read about Kevin Underwood. He kidnapped and murdered a 10 year old who lived in his apartment complex. Up to that point, he was merely depressed and lonely. No history of violence or subversive behavior. It was around 2006, so social media wasn't the monster it is today. A detective said if someone had seen his journal, and read some of his daily writings, they completely outlined his descent into madness and his plan to kill someone.
Not saying we should invade everyone's privacy, my point is a new habit or indicator has emerged, where most of these shooters will have the need to write about and publicize their intent. It's tied in with social media and our new mindset. Same way I feel the urge to comment and let my opinion be known here about space stories or sex scandals. There's an explanation in science, and it essentially says - We can't help but express our opinion in public because of conditioning. These freaks apparently can fall prey to expressing themselves in a similar fashion, and that's where we can start looking for clues to potential future crimes.