Unix is powerful. One wrong move in the terminal can disrupt an entire system. Here are a few of my favorite simple, beautiful, and chaotic commands (in no particular order):
$ rm -rf /
Deletes entire computer
$ :(){:|:&};
Creates a "Fork Bomb", which is a process that continuously replicates itself to no end; effectively depleting all of a system's OS resources and rendering it useless
$ chmod -R 777 /
Gives everyone and everything read/write/execute permission for everything on the machine
$ mv ~ /dev/null
Put your entire home directory in the "null" folder, essentially deleting it all.