Snape had only crossed half the intervening space when he saw the Hit Wizard go on alert. Under most circumstances the man's caution was admirable, but right now it was damn inconvenient.
Another complication was the fact that, though Snape was certain his quarry was Polyjuiced, he couldn't be certain that the impostor was Voldemort. It was unlikely, but not impossible, that his target was a simple gatecrasher, in the wrong place at the wrong time.
Fuck. I can't just take him down. As if the situation wasn't complicated enough, with far too many critically important targets in range of any flung curse.
The solution was simple, elegant. Excise the cancer.
Snape chose his angle of approach to give a clear shot on the target, though his gaze was fixed on MacLaggen. When he was in earshot he nodded meaningfully in Harry's direction. "Orta check wi' yer boss," he growled.
It would be good for barely a moment's distraction.
It was the work of a moment to fire a silent Petrificus Totalus at his quarry, while sidestepping so the rigid body was between him and the Hit Wizard, seize the man and Disapparate.