*[Serious] Redditors who have known people to have grown up to be serial killers/psychopaths, what were the telltale signs at a young age?

throwaway because i don't want my personal info out there, but someone i went to school with (we're in a tiny town where the most common crime is just teens dicking around and racing and public drunkenness) murdered a police officer and was later found out that he was planning to kill more people. i had maybe one or two classes with him, but he was always a very quiet kid and kept to himself. i'd say hi to him and be polite and stuff but i sort of kept my distance as he seemed a little off. here's the info of what happened:

On May 15th, 2004 at approximately 03:00 hours, Constable Chris Garrett of the Cobourg Police Service responded to a call involving the alleged robbery of an 18-year-old Cobourg resident. Constable Garrett was unaware that in fact there was no robbery and he was being lured into a planned ambush.

When Constable Garrett arrived he began taking the details of the alleged robbery and radioed his colleagues a description of a suspect. This left Constable Garrett alone with the supposed victim while the other officers searched for a suspect. While trying to help this apparent victim, Constable Garrett was suddenly attacked with a knife and suffered a mortal wound to his throat. Although dying, Constable Garrett somehow managed to draw his firearm and pursue his attacker, firing off his entire clip of 17 bullets. Constable Garrett was able to wound his murderer with his last shot before he succumbed to his injury.

Constable Garrett’s murderer, Troy Davey, was arrested at the Cobourg Hospital when he attended as a result of the gun shot injury.

It was later learned through the investigation that Davey’s actions that night were part of a larger, detailed scheme. After killing the police officer, he planned to steal a car and kill two attendants at a local gas station, then go to the Cobourg Police Station and set off two home-made anti-personnel explosive devices. He also planned to shoot police officers and civilian employees at the police station using a shot gun in his possession and the firearm stolen from the officer he had initially murdered.

/r/AskReddit Thread