How does roll return generate return? for the case of backwardation is it from purchasing new futures that were cheaper than the previous one such that there is a net positive from offsetting the previous?
It is future price - current price. In backwardation the future price is lower, so while the sign is -ve, the return is positive.
Think of it this way.
When the curve is downward slopping and you have to roll foward your future contract, you would pay less than current spot price, thus generating +ve return.