This should not be hard to understand.

The police find that man A kills man B. A is now the defendant in a criminal trial. The job of A's lawyer is to introduce facts that improve the outcome of the trial. Sometimes, that's fighting because there isn't enough evidence available to assert that man A actually killed man B. Other times, it's getting their client to plead guilty because it's the easiest thing to do in a case that they're guaranteed to lose. Other times, it's to get a lesser sentence because B was abusive to A and A couldn't escape. It could be that A was acting in self defense.

Removing all nuance and saying that the one and only goal is to get their clients out of trouble is incorrect. Not every defendant is guilty, and not every criminal needs the maximum punishment.