Sheriffs, district and states attorneys all run for reelection. They are incented to get an arrest and conviction. They don't always get things right.
Texas makes a good example, from the Justice Project.org:
9 people released from death row based on evidence of innocence? What level of error is acceptable when we are talking about the death penalty? To my mind it is none.
I have no problem with this guy and the Chuck Mansons of the world being executed. It seems very clear cut.
I have no faith that we can write a set of laws and procedures that works 100%. Imprisoning all evil folks for life seems better than executing a single innocent person.