Tip120 simulation shows explosion
You need a flyback diode (a diode across the load). 

When the transistor turns off (not on) the (collector) voltage will likely spike up to cause the transistor to break down, at least in your simulation. This is because the motor has some inductance. Inductance opposes a rapid change in current, by changing the voltage, so when the transistor starts to turn off the voltage at the collector rises to maintain the current until something stops it (in reality it might be transistor breakdown, or ringing from capacitance). 

Depends on how far from reality your motor model is. If it's just a resistor that won't happen in simulation. If it's a pure inductor + series resistor it will happen. Neither simplified model represents a motor load very well. 

A stalled motor will draw considerably in excess of its running current, because of back-EMF, for example.