The game at times seems to encourage you to attack others. For example, you have completed GE, or are not eligible for it, or GBG is not available or the attrition is ridiculous and you need to win so many battles. You are never required to plunder, as far as I know, unless possibly if plundering is normal for you. I never plunder except in revenge, but that is a valid game feature for those who need or enjoy it.