Is the fixed punishment approach the solution to crime?

Some say "Justice is blind", meaning that all of us should be treated in the same way by the system, and that the same crime should have the same sentence. Nevertheless, I think there are certain situations in each crime which must be taken into consideration during the trial and in the final sentence; and that can alleviate or toughen the penalty.
It is obvious that a crime does not always imply previous planning by the person committing it. The legal term manslaughter is an example of a crime committed without intention, and for me it is clear that the sentence should be lesser compared to the killing of a person that was planned by the murderer, because in the former case the person who committed the crime did not have the intention to do so.
On the other hand, I think that for minor offences the treatment should be the same for both; for the person who didn't have the intention of breaking the law and for the citizen who did it on purpose and wanted to get away with it. This is because when dealing with minor faults, fixed punishments work as an educational tool for correcting behaviours which are harmful to society.
While the fixed punishment is a popular point of view, some of us believe that everyone is different and behaves differently in the same situation, therefore this should always be pondered by the jury and the judge.
summing up, the circumstances of a crime, not only the crime, should be considered too.The legal system should take into account if there was a motivation for the crime or if the assassin planned it. These factors are as relevant as are the existence of extenuations, which make the legal processes so different, even when dealing with the death of a human being.