That's not the way it works. He's either sane, or he's insane. Insanity is going to be virtually impossible to prove as he fled the seen. Fleeing the seen is prima fascia evidence of consciousness of guilt.
His only hope is to plead to a second degree or manslaughter charge. But, if it goes to trial, I can promise you it will be first degree murder. Here's the law and minimums in Kansas...
Second Degree MurderSecond degree murder is an intentional killing that was not premeditated. Second degree murder can also be charged in instances where the killing was unintentional but reckless under circumstances manifesting extreme indifference to the value of human life.
The sentencing guidelines provide a range of sentence from a minimum of 147 months imprisonment to a maximum prison term of 653 months on an intentional killing, and a range of 109 months to a maximum of 493 months on an unintentional but reckless killing.
Voluntary Manslaughter
Voluntary Manslaughter is the intentional killing of a human being upon a sudden quarrel or in the heat of passion or upon the unreasonable but honest belief that circumstances existed that justified deadly force.
The sentencing guidelines provide a range of sentence from a minimum of 55 months imprisonment to a maximum prison term of 247 months.
Voluntary manslaughter is a distinct possibility. Second degree murder would be much more difficult.
What is the KS definition of involuntary manslaughter?