Hello,
You do not need to retain the internal resistance of a current source when it is 'killed'. The idea is to eliminate the effects of that current source completely while we look at the effects of the other sources. If you try to keep some part of that current source in the circuit while you look at the effects of the other sources then you havent killed it properly. To kill it properly it gets open circuited.
One way to look at this is to think of the current source as an infinite impedance as seen by the OTHER sources. For example, when we put a voltage source in parallel with a resistor and increase that voltage by some increment we see a corresponding change in current through the resistor, but when we put a voltage source in parallel with a current source and increase the voltage by some increment we do not see any change in current, and that's the same thing that happens when we put a voltage source in parallel with an infinite resistance and increase the voltage by some increment.