How to completely kill an internal drive?

1. Insert the Mac OS X Install disc that came with your computer, then restart the computer while holding the C key.
2. When your computer finishes starting up from the disc, choose Disk Utility from the Installer menu. (In Mac OS X 10.4 or later, you must select your language first.)
Important: Do not click Continue in the first screen of the Installer. If you do, you must restart from the disc again to access Disk Utility.
3. Click the Erase tab.
4. Click the disclosure triangle to the left of the hard drive icon to display the names of your hard disk volumes and partitions.
5. Select your Mac OS X volume, then choose MacOS Extended Journalled.
6. Click Erase & look for Security Options. *choose Zero 1 pass*.

Formatting & Partitioning a Hard Drive in OS X - Tiger and Leopard...

http://www.kenstone.net/fcphomepage/partitioningtiger.html

I'm in the Security options and notice there is also an option for "7-pass erase". Do you recommend the "Zero Out Data" because of the time it would take to choose the first option?

A single pass of zeros is sufficient. To get any more security, you have to open the drive and destroy the platters.

If you had access to a good condition SATA replacement hard disk drive
of the correct physical size and specifications to fit the computer at hand,
you could replace the hard disk drive and then reinstall a new OS X in it
from the installer disc set.

And then later on, dispose of the hard disk drive through destroying it,
if needs be; or by installing it into an external enclosure and then
using Disk Utility or some other drive utility (via any computer utility)
and have that other computer overwrite the hard disk drive with zeros.

The better way to overwrite for best success, is the longer duration
one, and it may take most of a day to do the task from Disk Utility.

Hopefully this helps. Are you close to a computer store where they
sell hard disk drives? Even a good used one of correct specs may do.

(I am not advocating doing anything illegal or untoward, but suggest
these ideas solely on the basis of containing personal information or
corporate intellectual properties you are responsible for, prior to their
being taken from your legal control.)

PS: I see you did get a reply, so that's good.

Best regards in this matter...
Good luck & happy computing! 🙂

edited.

Malcolm has you covered! 🙂