You cannot setup a turret after you're dead. Once you're down and the death animation kicks in nothing can be done other than call for help or sacrifice yourself in order to deploy again.
What happens, but many are so awful at this game that they cannot see it is this: The turret was being dropped as you shot the person and simply takes a minute to lock on, make the sound, and then attack. A truly skilled player notes this lapse between when the player initiates the turret setup and it actually finished setup, turns, locks on with sound notification, and THEN fires.
I'm far from the best player and even I noticed this in my first few games.
The ranger is also very noisy and sounds like a mechanical trashcan when it runs around. It also makes a noise before it fires on someone. Most people complaining about the ranger and turret (being "too strong") are not good players. I play this game and see how bad people are at it. Most aren't even aggressive at taking locations and act like they are scared to move in. That being said: Get GUD.