Mac died in the prison's gas chamber for murdering Brad's family.  Brad's crime was murdering Mac.  Using a chain saw, Brad cut down the only telephone pole near the prison, so the call ordering Mac's stay of execution could not go through. The two pieces of evidence were Brad's chain saw and the sawdust.

The killer is Brad Thompson.  His offence is criminal homicide. The murder weapon is Brad's chain saw.  Although Mac died in the gas chamber, the gas pellets were not the murder weapon.  The state did not commit homicide even though Mac's death was wrongfully carried out.