In absolutely egregious cases often the role of the lawyer is:
To make sure that the person actually gets their due process. Are their rights respected during the trial and sentencing. Do they get a fair hearing. This is important because if courts can bypass this for someone they can bypass it for everyone. Ensuring the worst of us get a fair trial protects the rest of society as much as it does the accused.
Cynically, especially where public defenders are used, part of the goal is to have a documented paper trail that all the i’s were dotted and t’s crossed to make sure the defendant got their due process to avoid them being able to make an appeal later on. This is also why some “obvious” cases take over a year to bring to trial - the state wants to make sure they don’t fuck it up.
Where the lawyer goes for a defense in these cases it is very often either to try to get a life sentence rather than the death penalty (sometimes due to personal opposition to the death penalty) or to get an insanity judgement (again mostly to avoid the death penalty) - this isn’t a pass to freedom, it’s a direct ticket to a secure psychiatric facility that basically a different type of jail where they can be held possibly forever.