To experience this sense of power one must 'know' that the victim is powerless. This requires empathy.
This doesn't require empathy! All the rapist needs to concern him/herself with is whether or not there is resistance. When an animal of prey goes after the helpless injured deer before trying to take down the healthy and strong one, do you think that's empathy? The animal (like the rapist) is looking for the path to their goal (food for the animal, power for the rapist) that will offer the least resistance.
If the animal attempts the deer likely to put up the greater amount of resistance and it gets away, the animal doesn't get its meal. If the rapist attempts to rape the individual which is puting up the greatest amount of resistance, he/she doesn't get his/her power (or at least not as much). The only real difference here is that the rapist can't decide which individual will put up the greater amount of resistance until he/she has already begun the act--though this can be true of the animal as well.
J0N