Barty's main intention with the Silencing Charm had been to prevent Marlene from being able to say any incantations -- of course she could still use non-verbal magic, but silent spells were both harder and, from what he knew, usually less powerful than when spoken -- although shutting her up was an added bonus about which he could not complain. He was just in the process of getting to his feet again when the streak of green light came rushing towards him, and in his instinctual panic, he threw himself down hard on the floor, narrowly missing being hit in his masked face. How the hell could someone like McKinnon be able to cast the Killing Curse, and how had she been able to do that when non-verbal magic was difficult enough, nevermind for a spell where an incantation was required -- oh.
Flustered and embarrassed that he had been tricked by some other spell that was undoubtedly harmless by comparison, he scrambled to his feet and flung a Reductor Curse at the sofa behind which Marlene had taken refuge, hoping it would explode and preparing himself to cast a Shield Charm to block any debris that was blown in his direction.