Green Lantern Tomar-Tu killed a Sinestro Corps member Romat-Ru even though the latter already surrendered. It’s not the first time a Green Lantern, or a Lost Lantern, killed a Sinestro Corps member who surrendered. Green Lantern Laira killed Yellow Lantern Amon-Sur after he killed Ke’haan’s, a deceased Green Lantern and Laira’s mentor, entire family. The shit eating grin that worm wore as he surrendered was infuriating, but what topped that was his taunt about escaping from prison.
That’s what villains do right? They escape imprisonment and commit crimes again.
Green Lanterns are authorized under special circumstances to deal lethal force. I suppose the line is drawn when your opponent has surrendered.