I don't know whether he'll be convicted of first degree murder, but what he did is comparable to the behavior of Jerome Ersland, the pharmacist who shot an armed robber after he (the robber) was injured and clearly no longer a threat; Mr. Ersland
was convicted of first degree murder.
From a
CBS report on the case:
Prosecutors say as Brady descended the basement steps, Smith shot him in the chest, then in the back while Brady fell, Wartner said. Smith fired a final shot into Brady's head, the bullet passing through Brady's hand, Wartner said.
<snip>
Smith dragged Kifer's body into the workshop and laid it on top of Brady's, Wartner said. Smith told investigators he thought he heard Kifer gasping, so he placed his revolver under her chin and fired what he told police was a "good clean finishing shot to the head," the assistant prosecutor said.
Among the similarities in the two cases: Mr. Smith, like Mr. Ersland, used a second gun to dispatch Ms. Kifer. He also considered shot placement before doing so, telling police that he shot her under the chin because .22 "doesn't go through bone very well."
I'd say there's a degree of premeditation there...