New option!
Add this line to your config.yml, if you don't want to lose your settings (from previous versions):
health-decrease-per-second-when-full: 20
When players reach 100% of fear, they will lose 20 hp/sec or 1 hp/tick, until they find a lighted space or die
Set it to 400 for an instant kill (because it will take 400/20 = 20 hp/tick).
P.S. It will ignore yellow hearts of absorption (because it's not a physical damage, but a mental damage

).
+Small bug fix with the /set-fear command