Going back to the justice argument, Marcia thinks that one man desrves to go to jail more than the other. She doesn't know that one man is completely innocent and the other is guilty. In fact, in this particular case, she is prepared to frame a man who she believes is innocent in this particular case but is (in her opinion) a bad man who deserves to go to jail.
On the other hand, she is prepared to allow the man who was actually responsible for the death in this case to go free because in her opinion he is a good man who has made a mistake.

I'm sorry but the last time I checked, police are not employed to make value judgements like this. They are employed to enforce the law and do so with impartiallity. That means that they must apply the law to everyone equally, without predjudice and without making value judgements.

It is the job of the courts to decide guilt or innocence based upon the evidence presented to it regarding only the case before it. That is why Justice is portrayed as being blind. We cannot allow the police to break the law (perverting the course of justice) in order to 'get' the people that they think should be behind bars. If Marcia really wants the bad guy in jail, then she should work harder to find real evidence of criminal activity on his part.

Too much of this type of thing goes on the real world already. If we as a society condone it and 'allow' our police to openly create false evidence against who ever they think 'deserves' to be punished, then we have destroyed the presumption of innocence and the right to a fair trial. In those circumstances, no-one is safe and everyone would live in fear of being jailed on false accusations at any time.