It seems that setting the speed to negative only works if the setting 'set angle' is disabled or that the object that spawns the bullet has itself an angle of zero. Don't know if this is a bug.
If you want that your bullet flies in the opposite direction that the object is facing you need to turn the object that the bullet spawns.
you could use in your second example: 'Sprite| Rotate 180 degrees (counter-)clockwise' when the condition triggers where you want to shoot the bullet in the opposite direction.