[PvK]

Like many such "moral" questions, the reason for doing the right thing is often not just "to be good" or even "to be able to live with oneself".

In this example, commiting attrocities as the occupied underdog resistance fighter may be very likely to result in terrible retribution upon one's own people (a typical historical result) - likely worse than non-attrocious resistance would. When the enemy has your people at their mercy, it's not the best time to play "who can inflict the worst attrocities". Even if somehow the enemy would not reciprocate, the tactical value of attrocity is often minimal, or even negative. In fact, a common and ancient propaganda technique (though also, not a wise one in the end) is to lie about the attrocities of the enemy, to promote fearlessness and determination in one's own people.

PvK