Hello seaserpentdesign,
Thank you for using the Apple Support Communities. I know having an issue updating your Mac is not what you expect. If I understand correctly, you are unable to finish downloading the latest update. These steps can help:
1. Use "safe mode" to start up your Mac so that it performs certain checks, delete system cache files, verifies your startup disk, and attempts to repair directory issues if needed. Log into your user account while in safe mode and try installing the update in safe mode. Then restart your Mac normally.
Try safe mode if your Mac doesn't finish starting up
Limited graphics performance in OS X recovery or safe mode
2. You can figure out if unexpected behavior like this is related to a user file or setting by trying to reproduce the issue from another user account. This process includes creating a new user account, logging in to it, and try updating in the new user.
How to test an issue in another user account on your Mac
Cheers!