High Sierra 10.13.6 - Security update 2019-003 failing install
Well it looks like following this should resolve the issue but its not for the faint of heart...

https://davidsimpson.me/2018/11/14/fixing-failed-macox-10.13.6-security-update-2018-002/

Unfortunately the issue doesn't appear to be simple.

Using terminal and running these commands seem to reset the update so it quits trying to install on shutdown.

sudo rm -f /Library/Preferences/com.apple.loginwindow.plist
sudo rm -f /private/var/db/.AppleUpgrade
sudo rm -f /private/var/db/.SoftwareUpdateAtLogout
sudo reboot


Black Screen With Cursor on Logout in Yosemite