Killing Salem/Brian/Keith lead to different routes, for example if you will decide to kill Brian you will get Leo route IN Flynn route, etc. for others.
Flynn doesn't have to have any reason to kill any character, he is already a monster changed by histeria, don't expect from him being this gila we knew.
Socket monster is able to travel through time, he is and isn't everywhere and in everyone routes and lives (I confused myself writing this one, I dont speak english, leme explain more: at the very same time he is at the lake when Syd got killed, when Jenna was upset and sad etc.)
In Leo's route lynx is confused instead of shocked, because he went through breakdown, he is broken to the core and in Flynn's he is more stable (that's why he catch more from monster)
Hope it has sense and that I helped :)