If you are too attached to abstract ideas of "Good," to the point of standing on principle even if it means defeat, then you may get defeated and replaced by someone worse, and things may be much worse for everyone if you did whatever was necessary to win.

However, if you are too attached to the evil approach, then you may become too caught up in self-indulgent antihero fantasies and lose sight of what actually works. For example, no actionable information was acquired through the use of torture during the War on Terror. Movies and TV shows almost always depict it working because that self-indulgent antihero fantasy sells, but that doesn't mean it's true. Violating international law could also (depending on the rest of the factions in this hypothetical) cause diplomatic repercussions such as sanctions or even another faction joining the war on the other side. Evil doesn't always get away unpunished.

Every faction, (as well as every person), has a variety of different approaches that they could apply, and what determines which of these approaches work and don't work is the system in which they exist and their position in it. "It is not the strongest of the species that survive, nor the most intelligent, but the ones most adaptable to change."