Why do you assume the defendant is guilty? What if he's innocent, but your refusal to help provide a strong defense causes him to be found guilty? Why do you presume to do the judge and jury's job for them, but with no access to the facts or judicial procedure?
If prosecutors can ensure that a strong defense is impossible simply by accusing someone of an appropriately heinous crime, then the criminal justice system has failed. Please don't be complicit in that.