In which John is a cuddly, BAMF assassin. (слэш)

This is a fill for this prompt, found here.
"Sherlock learns that a spat of recent killings was done by a professional assassin. A man so feared that the criminal underworld is afraid to whisper his name. Sherlock goes off on his own to track him down, and is captured.
Sherlock braces himself for a cold-blooded killer, with nerves of steel and no remorse, with a reputation for ruthlessness that makes several hardened criminals check their closets before they go to bed - and meets John. Easygoing, good-natured, cuddly jumper-wearing John. Who thinks he's brilliant and hilarious, and can he take him out for a coffee?
Cue the weirdest courtship ever. Like, dates being crashed by Russian assassins, epic gunfights in the middle of London, Lestrade and the Yard going WTF, Mycroft questioning his brother's taste in men, etc."