Prosecutors, in theory, are supposed to seek justice, not victories. (Defense lawyers, on the other hand, are supposed to give their client the best possible representation. If the defense gets a guilty guy off, that's the prosecution's fault.)
Prosecutors shouldn't bring a case unless both a) they think the guy did it, and b) they think they are likely to win. We can debate to what threshold of doubt each of those statements hold, but that's the general gist.
Prosecutors are supposed to zealously represent their client, which in this case is the state. The are required to seek "justice" but necessarily that is defined somewhat more narrowly than in common parlance. They are not supposed to substitute their sense of justice (one way or another) for the state's and the people's sense of justice.
Drug laws are really the best example of this. Prosecutors are not allowed to say "I won't prosecute this case because drug usage is a victimless crime." My mom thinks drug usage is contagious in social circles and a tremendous danger to the youth, votes accordingly, and until that changes it's not right for some unelected prosecutor to second guess her.
Prosecutors shouldn't bring a case unless both a) they think the guy did it, and b) they think they are likely to win. We can debate to what threshold of doubt each of those statements hold, but that's the general gist.