How do I completely remove Avira?

What backups do you have?

Assuming the answer is "none" create a Time Machine or equivalent backup first. Until then, make no other changes to your Mac's file system.

Then: Restart your Mac in "Safe Mode". That will prevent all the processes associated with "Avira" from launching at boot. Repeat the manual removal technique you described, bearing in mind that it may result in a system that is unstable or cannot be started. Your recourse in that event would be to completely erase and restore your Mac from the Time Machine backup.

Then: never install any non-Apple "anti-virus" garbage, ever again.

For what it's worth these are the files and processes associated with the version of "Avira" I installed for evaluation purposes. Please bear in mind that it may not be a complete list or apply to the version that you installed.

Uninstaller app and one folder in Utilities

~/Library/Application Support/Avira

/Library/Application Support/Avira

/Library/LaunchAgents/com.avira.antivirus.ipm.ui.plist

/Library/LaunchAgents/com.avira.antivirus.odscan.default.plist

/Library/LaunchAgents/com.avira.antivirus.systray.plist

/Library/LaunchAgents/com.avira.antivirus.update.default.plist

/Library/LaunchDaemons/com.avira.antivirus.dbcleaner.plist

/Library/LaunchDaemons/com.avira.antivirus.ipm.loader.plist

/Library/LaunchDaemons/com.avira.helper.watchdox.plist

~/Library/Saved Application State/com.avira.controlcenter.savedState

kernel extensions:

com.avira.kext.FileAccessControl

logs:

/var/log/com.avira.helper.watchdox.log

processes:

AV_Systray

savapi

avguard.bin

avguard-ondemand.mgmt x2

avguard-scanner (many)

avlogwriter

avqmd

avasavapi-super

avwatchdox.bin

avbackendproxy worker (avguard-license) (many)

avbackendproxy worker (update)

After using its uninstaller the following files and folders will remain and require manual removal

~/Library/Application Support/Avira

/Library/Application Support/Avira

~/Library/Saved Application State/com.avira.uninstall.savedState

~/Library/Saved Application State/com.avira.controlcenter.savedState

/var/log/com.avira.helper.watchdox.log

Good luck.

You will have to go with the manual removal now.

Or reinstall Avira and then use Avira's uninstaller