Secure erase drive in Single User Mode
Normally from the command line, you would use the diskutil command with the zeroDisk, randomDisk or secureErase options to securely wipe a disk. However, I would imagine this doesn't work on the disk you've just booted from. So I suspect you'll either have to find another Mac and connect yours in target disk mode via a Firewire cable, or physically remove the drive and connect it to another Mac using a disk caddy, to wipe it the way you wish.