[It's not until Porthos is right behind him that Sebastian seems to notice he's drawn up that close, and he starts to turn around--
(if he's about to be shot he'll have to intervene because he's not in the mood to cough up bullet splinters. Ah, but no, he's going to be bludgeoned? Far less of a hassle. Sebastian's first instinct is to stop it, of course, he's in no mood for this nonsense, but-- maybe this is a convenient way to finally have some peace to go about his business alone. Let Porthos knock him out, blow off some steam and be the hero, and this won't even have to turn ugly - for Porthos - later.)
--but he's not fast enough, and the strike connects, if slightly to the side of his skull because he moved. Startled expression still on his face, Sebastian staggers back, only making a faint noise of surprise before he sags to the ground. And then remains lying there.
(Now if Porthos actually intends to beat him to death he'll have to put his foot down, but one thing at a time.)]