Actually, CTRL-ALT-DEL is an absolutely acceptable way to shut down Slackware. The key combination and resulting action is actually located in the /etc/inittab file and simple calls shutdown for you. Although, by default, I believe it actually reboots the computer instead of simply shutting it down. Either way, you can customize the entry in inittab to do whatever you'd like.
Post the error message you are getting when doing a 'init 0' and maybe someone can shed some light on the mysterious message.