The way I read this is that the murderer held his breath and thrashed for 2 minutes, then out of an abundance of caution they waited another 20 to declare him dead.
And the moron probably spent the last weeks, or perhaps months of his life, practicing how long he could hold his breath.
.357 magnum to the head. Lights out and only a buck 20 to the taxpayers. Prolly too good for the worthless MFr but justice served.