Is killing people really necessary? Would a choice between slavery and death by starvation do?

Make the criminals work on something and then don't give them food unless they produce.

I do not know what that something could be. Maybe start from diggind ditches chained to a irom ball and then if they behave they could get a better job.

They would never become free but they could get better jobs and food. Not working at all would result in no food.