And that only happens above 8000 ft. The aircraft may be "tight" and lightly pressureized when the door closes, it truely isn't pressurized until above 8000 ft.
I am no marksmen, but I am a good shot. a plane accelerating to altitude is cruizing up to 350mph, who would be able to land a killing shot?
You would have to have close to a head-on angle. And I'm betting a silenced/suppressed 50 isn't going to have the range required, would it? So if they're firing it unsuppressed, this wouldn't be an operation they could carry out and then sneak away from too terribly easily. It *is* possible, in my mind (they would probably have to leave the weapon behind and/or fire from a deeply wooded area and then hightail it out of there fast.) They could also do it during deer season, to cover the sound, but then they'd run a higher risk of running into other hunters and having to kill a few more people to escape.
It would seem fairly risky and would almost certainly result in being caught. But is that a concern to them?