The judge didn't let him go because he did a good thing. A mistrial isn't a not guilty verdict. It's basically just saying, "This trial is fucked beyond repair, try again."
The judge ordered a mistrial because the other jurors would likely be swayed by him saving another juror's life and rule in his favor despite evidence of his guilt.
The case went to arbitration after that.
Regardless, a mistrial doesn't mean the defendant is off the hook.