KillDisk is a DOS program that will kill ALL information on any mounted disk. It will not work on a Mac.
The reason that this machine is having great difficulty rendering websites is because it is using a very old version of the Mac OS, Internet Explorer, Java Runtime. Basically everything is out of date.
I guess the real question is; what are you trying to achieve with the wipe out of the disk?
This old iMac, if it is as old as you suggest, probably has 32 or 64 MB of Ram and a 4 Gb hard drive. You indicated that it is running OS 9 which has not been updated for the last 7 years. While this machine will do email easily, it will not perform well on most websites.
To find out exactly how much RAM it has, HD size, processor speed, etc., you can check using the System Profiler. You will find this under the Apple Menu, top left corner of the screen.
On the subject of erasing a drive using Shredit or KillDisk. The computer normally uses an internal HD to startup from (Boot), by reading the Operating System that is installed on it. (OS 9, in this case)
Once the machine has started up, it will not allow you to remove the Operating System from the HD because the computer is using it to run.
To overcome this, you have to boot the machine from an alternative source. External HD, Floppy disk (Not fitted on Mac's) or CD.
Now the OS on the HD is not being used and the entire drive can be erased without a problem.
Once again, before you do this dastardly deed, what is the overall plan?
P