Why would you think that?
Just because his statement is not admissible, it doesn't mean there is no case against him.
There is still the forensic evidence that the bullet came from his gun, there is his partner's testimony, and the process of elimination that it is most clearly he who pulled the trigger.
They don't need his testimony to try him for at least manslaughter. I'd think the best that his own testimony would provide is evidence of 1st or 2nd degree murder, which implies intent or reckless disregard.
-PJ