[ryanmercer]

Or they know the individual and feel slighted by them somehow and, like unwell people are tend to do, got obsessive and went long-term.

Humans are odd creatures, it's entirely believable to think this person didn't wish them happy birthday x years ago or cut them in line at Starbucks and the person made it their mission to 'exact revenge'.

A former co-worker of mine was angry over another co-worker over something trivial. She had a known peanut allergy and at some point someone coated her keyboard and desk with peanut butter powder (which was blatantly obvious that something was all over her desk). We all knew he did it but no one saw it happen so nothing could be done about it. He would also go on and on about "you don't want to be on my list! I'm going to put you on my list!" but he'd never say it around management so again, they wouldn't do anything about him.