You can kill the x in Ubuntu with dontzap. Its really convenient if you like it. First thing you need to do is to download this package called dontzap. Now you can run the command sudo dontzap -d, it will disable that feature that Ubuntu uses and it will let you kill x by just pressing the ctrl + alt + backspace. It is really simple as that and there is no chance that you accidently pres the ctrl + alt + backspace and kill your x session. So after that you need to reboot your computer and you are done. Now whenever you want to kill you x session just press the ctrl + alt + backspace and that's it.