depending on the circumstances, I do think a prosecutor can play a pivotal role. There are examples of misconduct where a prosecutor fails to turn over exculpatory evidence that is so convincing as to make prosecution essentially impossible. It's true that the jury and judge make the decision, but there are cases where they would almost certainly have reached a different decision had the prosecution behaved ethically.
"It is as much his duty to refrain from improper methods calculated to produce a wrongful conviction as it is to use every legitimate means to bring about a just one."
Assuming the prosecutor believed the man to be guilty, and that the prosecutor didn't use improper methods to secure a conviction, I'd say they were competent.
Now, if they used _improper methods _to secure the conviction of an innocent man, or if they knew the man to be innocent, that's another matter. I'd agree that the prosecutor is in incompetent (in the sense of "not fit to be a officer of the court", and not in the sense of "not having the skills required to do the job").
But the post to which I replied made no mention of this evil intent, so I gave the prosecutor the benefit of the doubt.
That man was put behind bars by an incompetent prosecutor... who had become a lawyer without going to law school.
(Seriously.)