I've been in this field for a long time and have never had an issue like this. The long and short of it is that in three different instances I was installing software on my system.
An MS MSI update, Starcraft 2 and an NVDIA driver update. In all three cases the system shutdown during software load; not rebooted. The system shutdown.
I can (and have) run other pieces of software (gaming and otherwise) without issue for days on end.
1) Verified that the system is not overheating (just so that I could sleep at night.)
2) Verified that the system was virus/malware free.
3) Disabled antivirus during software installation attemtps.
4) Verified that this is not a profile issue.
5) Event logs show nothing definitive. (Services started, etc.)
6) Updated video drivers (which caused a shutdown as well).
7) Verified that system board, was up to date for chipset and BIOS releases.
8) Ran a Repair (No joy).
Now there may be other pieces of software in my future that will cause the same shutdown. But at this point it is only the three I mentioned above. Other installations from the web or from media install and run as designed.