Sebastian waited until they were out of earshot from John upstairs and forcefully removed his wrists out of Sherlock's grasp, turning round and punching him in the face--giving him enough time to grab the switch blade out of his pocket.
He violently grabbed the startled detective from behind, cupped one hand over his mouth to stop any noise being heard and held the knife against his throat threateningly.
"If you dare say anything... I will slit your throat and watch you bleed to death." he hissed in his ear, pressing the blade harder against his throat. "Would you like that?"