Greg rolls out of the way of the slayer's silver stake, and Molly intercepts the woman coming at him from the other side, twisting her arm back behind her so that she drops the stake. The slayer attempts to pull away, but Molly tightens her grip, an automatic reflex left over from the times she's wrestled bottles of cocaine off of Sherlock.
The first slayer makes another attempt on Greg, but again the werewolf is too fast for him, twisting around and embedding the stake instead deep in the slayer's abdomen. A gurgling noise, an attempt to pull the stake out. And slayer though he may be, it's still against Greg's nature to let the man suffer to death, so he pulls his pistol and presses it to his temple, ending it as easy as that with a spray of blood and brain matter against the alley wall.
By this time, Molly has knocked her would-be assassin into unconsciousness, with a carefully calculated clocking of a pistol to her forehead, outright killing being as against her nature now as ever before.
"You okay?" Greg asks, eyes filled with concern for the pathologist. (They've known each other for years, and it's never gotten beyond friendship, but sometimes when he sees her face flushed like it is now, eyes twinkling in the heat of the moment, he finds himself wishing that it was.)
Molly smiles back at him, catching her breath and leaning against the cold stone wall, slipping her gun back into her waistband. "As I ever am."
Silently, they tie the living slayer, leaving her ready for Mycroft's pick-up crew. The dead one they leave as he is, unable to harm anyone now and certainly unable to threaten them or their friends, almost family.
Without questioning it, they move on, Molly leading the way, the ghosts of the past leading them into the ever-approaching dawn.