Spike knew that the chances were fifty-fifty as to whether the nod was a distraction or a genuine warning that there was something - or someone - to see. He took his chances and zeroed his aim on the stocker's chest instead of looking behind him. Even if there was something back there, he'd just be doubly screwed over if he took his focus off the initial target. This was basic common sense as far as he was concerned...
The female voice came mere seconds after he pulled the hammer back - an unnecessary move turned into a gesture of assurance that he'd not fall for the 'look behind you' trick. Too bad it wasn't much of a trick. His expression darkened in response to her words, becoming less borderline-playful and much more antagonistic.
Spike stood his ground silently, staring down the male as if daring him to move. He didn't speak his refusal to surrender, as it was obvious enough without ruining the moment - or his own concentration. But he also didn't shoot just yet. It was nearly like a twisted game in his mind, the way he allowed a clash of wills to develop. A challenge to try stopping him...
What did he care if everyone died from lack of supplies? What did he care if he had to fight off an entire pseudo-colony of angry people? He could handle it. He'd handled worse before. Or maybe he was just too stubborn to think rationally in regards to the plethora of possible consequences. After all, when he'd handled worse, it had been with much more than a single, 9mm pistol... and with a lot more ammo.