15 to 20 more likely, or 5 to 10 if he acts like a crazy man convincingly
I've never understood why mental illness is a defense. I mean, any murderer has to be a little fucked up in the head, doesn't he? Should we just let all murderers off easy?
My view of the law is that it serves three purposes with three levels of importance:
1) Protect the public by removing the threat.
2) Rehabilitate so that upon release the perp is no longer a threat.
3) Punish the perp for the crime and deter other people from committing it.
Mental illness only comes into relevance in #3 in that the argument could be made that they weren't fully aware of what they were doing. #1 and #2 are far more important in my mind.