Why is it negative for -Vout / Rd ?

 

Do you mean "why is the output voltage inverted with respect to the input voltage"?
That's because an increase in input voltage causes an increase in the output current.
This then causes the output voltage at the resistor collector junction to decrease.

 

Vout is negative because gm*vgs current source "sink" the current from RD resistor. Note the gm*vgs direction and this mean that RD current will flow from reference point (GND) up through RD resistor (from bottom to top ) into gm*vgs current source down through RS to GND.

 

Basically your current (gm * V1) is flowing in the opposite direction from what you are taking your voltage reading across: resistor RD.