That's the only posible end of Flynn route and there only one thing wich we can influence on, by choising who's story you want to be completed: Flynn's, Leo's or ... Jenna's maybe ? And I dont think fully possesed Flynn was the killer more likely that *That* thing gave him
all the answers by warping him through time, space and even multiverse's of Echo (as we seen before with Chase's car, it's posible)
and mutilated Flynn" just been an observer of events of the past(only proving point that all going in circles).