WA does not codify MS as either voluntary or involuntary:
It is simply titled: Manslaughter
http://apps.leg.wa.gov/RCW/default.aspx?cite=9A.32.060
http://apps.leg.wa.gov/RCW/default.aspx?cite=9A.32.070
It appears he is going to be Indicted on both homicide charges. I do not know if MS is a "lesser included" offense for Murder under WA law or not? I would greatly surmise, yes!
Regardless, the state has some procedural reason for charging both, either on arrest or subsequent Indictment, if he has not been Indicted yet.
Lesser offenses are requested by the defense as a jury instruction and can be considered just as the original charge in the Indictment.
This is why it is imperative an attorney versed in criminal law represent him. It is thier job to decipher such criminal procedure elements.

