The dead body was the least of their worries. Frankly, it was the wooden stake in Sherlock's thigh that was the most troubling, closely followed by the approaching dawn. It would be sunrise in seven minutes, and it would take them about six minutes to get back to Baker Street. Of course, that was at full run. And without a stupid leg injury.
In John's own words, they were rightly fucked. (Sherlock, of course, said that he could arrange for them to be rightly fucked if John would only give him five minutes. John swiftly put an end to that plan.) He didn't relish the thought of being caught outside in daylight, and nor could he say that he wanted them to be found with the body of a notable slayer either.
Nights like this, he wished he was back in the army.
Every second they stood deliberating their options was wasting more time. So they took off, keeping to shadows for the most part, silently deciding that if the worst came to the worst, they could always take the chance and risk hiding in somebody's cellar.
It didn't come to that, though. About a block from Baker Street, John hauled Sherlock over his shoulder and ran. When they tumbled through the door, each knew it was good to be back.