Author's Note: I've had this idea for quite a long time and I finally wrote it out on Twitter so now I'll post it here.
Sherlock always had been interested in murders- those were always the most interesting and complicated crimes to investigate. He couldn't help but imagine himself in each position every time he was at the scene of a crime. There was something intimate, he felt, between a murderer and his victim. Guns didn't count. Guns were nothing. But other things, such as strangulation... Crawling over another human being, hands around their neck. He closed his eyes as he put himself in the murderer's shoes. If he were to kill John, which he never would do, of course, this is the method he would choose. He would press his hands to John's throat and kiss him so he could feel his last breaths, making them his own. He wouldn't watch his eyes. He knew it would frighten John and Sherlock could not handle that. He would know he wasn't doing something "bad" necessarily. He knew John was hurting. His whole life was filled with pain. Sherlock would be with him in his last moments, doing his best to reassure him as he literally took his breath away.