Go Galt. The Big Bad Sheriff of Nottingham can't take from the peasants what they don't have. And it doesn't avail the king much to starve the peasants to death; all he ends up with then is a disposal problem and nobody to rule.
The peasants outnumber the king's soldiers, and the king has to feed, equip, and shelter them -- at great cost to his treasury.
Or stand together and fight. Ambush the king's columns as they march on the villages. Infiltrate his court and poison a few of his advisers. Drag the sheriff out of his manor and hang him from a tree. But don't delude yourself by thinking that stealing from the thieves makes you anything but a thief yourself.
So theft is wrong, but murder is ok?