How to guarantee a clean install of macOS Mojave from an external USB drive I recently received some Mac Mini's (2014 2.8 i5 16GB RAM) purchased online which came installed with El Capitain.  I was concerned that the there could be a root kit installed on these second-hand refurbished machines.  So I wanted to do a clean install of the OS.  I followed instructions (https://www.macrumors.com/how-to/macos-sierra-clean-install/) to create a bootable version of Sierra and restarted the machine with the option and reinstalled the OS from the USB disk.  

To my surprise, when the system restarted after the "clean" install, the user that was created before the "clean" install was still there and so was all their user preferences and applications installed.

Is this normal for a "clean" install from USB?  What prevents a re-seller from installing a root kit in the user's directory that may include a cron, or even another sub-system that would still be there after this "clean" install?
Like Allan said in his comment that step specifically says


  Once the USB drive has booted, select "Disk Utility" from the Utilities window, choose your Mac's startup drive from the list, and click "Erase". 


This is one of the most vital steps in the process of a clean install. It COMPLETELY wipes the target drive of all information previously there. If you don't do that all you are doing is reinstalling macOS right over top of what is already there leaving everything that was on the hard drive still there.

If you do that there will be nothing left, no users, 3rd party apps, malware, etc.