You ready your shield, then proceed to pummel the prisoner into unconsciousness. He falls to the floor, to all appearances lifeless. You're familiar enough with this manuever that you know you haven't killed the man, but the Defias has no guarantees, so you check the man's pulse to give yourself more plausibility when you announce, "He's dead." Vanessa looks mildly convinced. "Good," she says. "Consider your rite of induction into the Defias passed." A couple of guards come over to dispose of the Stormwind officer's body.
See if you can stop the guards and "dispose" of the Stormwind officer personally. If you want to save this man's life, better not to take the chance of "disposal" being lethal. Don't try to stop the guards. Looks like the odds that you can get away with this now and live are low. Just cut your losses and wait for your next order.