Was justice in the middle ages harsh and superstitious?
Most life in the Middle Ages was on the edge, on the edge of starvation, on the edge of freezing, on the edge of dying of disease. There wasn't a lot of food to spare. If you stole something, you may as well have killed him because it would be difficult to continue life without it. Horse-stealing was especially bad, because if you stole a horse, you were probably stealing a man's livelihood, his work, and his transportation all at once. Additionally, there were few prisons, no jails, and no way to house or feed a person who wasn't earning his own keep. And only the King had men to spare to keep someone like that alive and imprisoned.
So most crimes were capital crimes were punished by death.