Blood
The corpse is a young man, a boy really, athletically gifted but yearning for a career in the arts. If his parents had been less intent on forcing him into their narrow conception of success, he wouldn't have made himself a target by resorting to the subterfuge that let him participate in the local theatre troupe.
"The killer mistook the victim for gay," Sherlock informs Lestrade.
"He wasn't?"
"No. He just liked acting. The killer misread the motives for his furtiveness."
Sherlock can see tension snapping across John's shoulders where he's crouched over the body, though no one else is bothering to pay him any attention at all. They're all focused on Sherlock.
He adores it. Everyone always worries about Sherlock, when between the two of them, he's not the one who 's proven himself capable of killing. Lestrade is so lucky they aren't inclined toward crime. Between Sherlock's mind and John's ordinariness, they'd be unstoppable.
Just now, though, what no one else is bothering to spot about John is that something about this murder has its claws into him. The victim suffered significant burns; the obvious answer would be Afghanistan, but…no. The victim's youth…ah, Harry.
Of course. The killer is targeting youths of heterodox sexuality. The stresses on John's relationship with his sister are old and well-worn, growing out of the drinking which most likely dates from when Harry first came to grapple with her sexual preferences, typically puberty or young adulthood. If he were pressed, Sherlock would place it at puberty, given the siblings' behavior patterns. She was young enough that John was present for the beginning of it, which as she's older by three years means she'd not yet left for college. Which means that he grew up watching her struggle with her sexuality and endure discrimination for it. Probably faced some himself as well, by extension.
In short, John is imagining his sister burned and bled out on the floor.
Lestrade is babbling nonsense that's better left unheard, so Sherlock leaves him to it. He walks over to stand before John, who sits back on his haunches and looks straight up at him.
Sherlock doesn't get along with his sibling, either, but that doesn't mean he wouldn't eviscerate anyone who messed with him. In the extremely hypothetical case that Mycroft wouldn't have already peeled them and hung them on hooks in some eminently deniable dark room, of course.
Wait. Deniable dark room… Oh!
He smiles down at John with what he knows is a wolfish show of teeth. "I know where the bastard's operating from."
John's eyes kindle.