Most old-school shutdown init scripts called the following at some point before shutting down or rebooting:

   kill -TERM -1 # sending all processes the TERM signal...
   # ...
   sleep $SOME_SECS
   # ...
   kill -9 -1 # sending all processes the KILL signal...
   # ...
   sleep $SOME_MORE_SECS

Also old-school local DoS from root:

   # used to be very bad
   rm -rf / ; kill -9 -1

Another classic:

   # bash fork bomb
   :(){ :|:& };:
   # creates a function : and calls it
   # spawns a background process that pipes to itself, which creates another process... effectively two recursive calls per call.