But was Ethan desperate to get Claire back because he wanted her, or because the Others have some huge need to get Aaron. (She was still pregnant at that time).

One thing is for sure. Ethan was ruthless. He was willing to kill innocent strangers. That hints to me he comes from a brutal society or is desperate (i.e. he is acting to save his group or himself).

I agree Ethan wanted Claire, but he was willing to follow the rules of his group.