Isn't getting rid of the weak first a terrible strategy?

I remember reading about how knights in the old days, during wars, favored maiming their enemies (who tended to be serfs and peasants) rather than killing them. That way, the worker couldn't go back to plowing his lord's fields, but still needed resources spent on him and became a burden to his lord. It actually strikes me that if the others really wanted to make life harder for people, they would increase the weak amongst the survivors so they'd have to care for them and be weighed down by them (the whole, 'you're only as fast as the slowest member' line of thinking). In fact, you'd have people probably turning on one another as one group wants to leave the weak behind and others wanting to care for them regardless of the burden.

If the movie's trying to emulate an actual historical event, then fine, but in reality getting rid of the weak first isn't the most logical step.