Yes, you've made your decision. And though it's a grim one, you know deep within what's left of your spirit that it's the right choice. Lucca turned on you; she was prepared to sacrifice you on the altar of her own self-interest. It's a betrayal that you simply can not abide. If you're ever going to strike, it should be now, while she's distracted and searching for a way back to the battle. Her back is turned.
It's now or never.
You slam your fist into the back of Lucca's head hard enough to snap a mortal's spine in half before she even realizes your intention. She starts to whirl to confront you, her reflexes lighting-quick but not fast enough to make up for being caught completely off-guard. You slam her up against the splintered concrete debris, skewering her on a sharp piece of rebar. She grunts as the metal tears through her body, narrowly missing her heart as it sprouts out from her back. She tries to wriggle free, but you grab the end of the bar and twist it back at an angle, impaling her like a fish on a hook.
"Are you kidding me?" she yells. "What the hell is wrong with you? I get it, you're mad, but this is too much!"
You don't respond. You have a grim deed to complete and you need to be decisive in how you go about it. Destroying a vampire is no simple business, and if you fail to finish the job, there's no doubt that she'll come after you and return the favor.