The guards have been watching the Septonian Pilot this whole time though, its a bit unrealistic that the Septonian can take out the first guard and get the second guard as well. As soon as the gun is pulled both guards are running for cover, the Septonian may have enough time to shoot one, but not enough time to kill the other.

Edit:I changed my post, it better explains my reasoning as to why the second guard could survive.

Edit 2: Edit Harder: I found the issue, I mistook the size of the weapon the Septonian was using and was wondering how he snuck a large weapon under the eyes of the guards, was actually smaller. Am fixing post.

Edit 3: And it is fixed