There's a thing worth learning here - and that is that Ctrl/Alt/Fn (where n is a number, from 1 to 6, or so) will switch you between "virtual consoles". Many new users don't know this.
So, two ideas/solutions related to the OP, which use virtual console switching:
1) From the GUI (desktop), if you press Ctrl/Alt/F2, you will get to virtual console #2 and it will look for all the world as if you have exited the GUI. This actually should suffice for most people's desire to "kill the desktop". You can then get back to the GUI (which is, in fact, still running), by pressing Ctrl/Alt/F7.
2) Or, if you really do need to kill it, you can press Ctrl/Alt/F1, which switches you to virtual console #1, and you'll see a bunch of messages that are the output from having run "starts" on that console. If you now press Ctrl/C, it will (gently) "crash" the GUI, returning you to the command prompt on that virtual console.
And some folks need to stop being fanboys and see the forest behind the trees.
(One of the best lines I've seen on this board lately)