Some years back I failed a PPL candidate for getting just under the required pass mark in an Aviation Law ground exam.
Debrief..pink slip and "better luck next time" and off we went for a coffee, after which he said goodbye and went home. That was that so I thought.
About 3 hours later he staggered back (very very pi$$ed) into the school and launched a viscious torrent of abuse at me culminating in a statement to the effect that he knew where I lived and was going to burn my house down!
Fortunately for me a couple of school members pitched up and "ejected" him...
Didn`t call the police but in the event I didn`t need to as he was picked shortly afterwards having driven his car into a wall on the way to wherever he was going.
Never saw him again - but alerted colleagues at other schools and clubs to watch out for him.
Presumably he had other issues going on - but prior to this as I recall nobody at the school had any reason to suspect anything 'dodgy' about his character.