Double Jeopardy

 נקי וצדיק אל תהרוג כי לא אצדיק רשע

This is a prohibition against double jeopardy meaning that once a defendant is found innocent he cannot be tried again. the Pasuk  can be explained as saying that it is unfair to try someone twice if he was found innocent the first time. Justice needs balance. If one is found guilty and after he is found guilty the court follows through on the punishment e.g. if he is put to death, there is no second chance for that defendant and even if he is exonerated at a later date there is no recourse. Being that that is the case for someone who is found guilty that has to be the case for someone who is found innocent too. You cannot have the possibility of double jeopardy on one side and not the other. That is what the Pasuk is saying. 

נקי וצדיק אל תהרוג - The court cannot judge an innocent man-innocent in the sense that he was already found innocent in court, The reason for this is:

כי לא אצדיק רשע- If someone was found guilty he has no recourse (after being punished) to be tried again and found innocent. 

justice needs balance and if a guilty man has no recourse then an innocent man should not be worse off..