Modern jets are inherently unstable without the vertical stabilizer. There's nothing to keep the nose pointed forward. The plane can simply rotate on a horizontal plane without any form of overriding control. Airplanes don't fly well sideways. With modern aircraft the fuselage extends as far in front of the wings as it does behind so it doesn't not act as a stabilizing force (if there's more fuselage in front than behind then it is a positive feedback system without a vertical stabilizer). Flight 587 lost the entire vertical stabilizer (not just the "rudder" flap) in flight.
“Modern jets are inherently unstable without the vertical stabilizer.”
You weren’t talking about the vertical strailizer: You were talking about the rudder, which is the movable, vertical “flap” at the trailing edge of the vertical stabilizer. The vertical stabilzer combats drift and yaw. The rudder, along with the ailerons, allows for turning.