Why would an NSA guy put secret government tools on his personal laptop?

Stupid as it may sound, but my experience with many many "why did you take the data there" dramas, the answer is:

To get things done you can not do at the office or you just lack the office time to get it done.

Too restrictive corporate policies?

User error

Why would a NSA guy even run any AV? Isolate and compartmentalize everything based on the task and its dependencies. You should assume everything you run could be bad or that you are already compromised.

He works for the NSA, but he was on his home computer which is unlikely to stay air-gapped unless he's content with making mspaint art and playing skifree :)

Straight up. They spew forth this stupid reasoning so that the general public will become frightened. Most people don't understand what any AV does, or how it operates anyway. For them to understand compartmentalization based on dependencies is way too far out there. The US government might have granted access as well in another effort to spread fear amongst the uneducated American populus.

Is this reasonable to do with number of softwares even average people use?

There was a person on the docker team, who had dockerized every other applications like chrome, firefox, ALSA sound server, and more. But even she found it hard to sandbox everything.

I'm using docker as a leading sandboxing tech. Do you mean something else when you mean sandbox?

I should warn that Docker was never planned as a security tool. If you read the documentation on Linux containers you will see that they are pretty complicated and therefore can have vulnerabilities.

Because he's a RIS mole pretending to be incompetent.