[G3 Q&A, Help & Troubleshooting] pol098:
While old, this is probably still relevant - I came here via a search, and found this to be the only way to resolve the McAfee lock. I was trying to downgrade an LG G3 D855 phone so that I could root it, and flashed a file that was supposed to be a stock ROM, D85510A_00.kdz, the oldest KitKat file for this phone. Big mistake, it had NcAfee installed and locked the phone. My McAfee was registered to an email address, and the only way to unlock it was to log on to "my" McAfee account, or have an email sent to this address. McAfee tech support had a very supportive attitude, but couldn't do anything as they didn't have the email address registered. They didn't know their own product too well, suggesting that if I managed to start in Safe Mode I could remove the McAfee app. Not so. Recovery and download modes reported that they had been disabled by the "kill switch". There was nothing I, or McAfee could do.

I came to this thread; the fundamental fact I learnt is that repeated rebooting eventually got the phone started without locking. It didn't work very well, presumably with McAfee using system resources as it tried to lock; it wasn't possible to install anything from the play store ("downloading" for 5 minutes or so until I gave up), and programs worked slowly and not very well with McAfee active and trying to do its thing.

The exact procedure listed here didn't work for me, but once I knew I could get into the phone by repeated restarting I could try anything; anyone reasonably familiar with Android should be able to cope with whatever the system throws up. I first installed Towelroot 3.0, which rooted the phone without needing a reboot I installed files by writing the .apks to the SD card on a computer. I installed the System 2 task manager, Titanium Backup application, and SuperSU at various times. I also installed Total Commander, my preferred file manager, though the built-in one might have been enough. The first thing to do to stop McAfee from undoing everything, once rooted to provide access to its file /system/priv-app/LGMcAfee.MobileSecurity.apk, was to stop the process with System2. (Until then I had done various things, but later found that McAfee had managed to undo them and resuscitate itself on next reboot.) After stopping the process, I deleted McAfee with Titanium Backup. That's the essential unlocking bit. I also installed the TWRP app and used it to flash the TWRP recovery. So I had a phone working well, rooted, with the earliest release of KitKat. Booting in factory reset mode and ignoring the warnings that everything will be deleted starts TWRP in the way I've been used to, and allowed me to restore a backup of the phone (or install a custom ROM).

[When repeatedly rebooting until McAfee doesn't kill, find something not very taxing to do, e.g. watch a video., so the time is not entirely wasted.]

[Added later]: instead of repeated rebooting, it's been suggested that if you manage to go to Settings > General after the boot process has finished but before McAfee has loaded (about 5"), McAfee will not fully start and you can do the root, terminate, remove process. As before the phone will be sluggish and not work properly, and the Play Store will not work, until McAfee is terminated. Put .apks on an SD card, or use a browser to download them from the phone.