this scene peter neal is actually an innocent witness who was crouching in the bushes nearby the whole time and spying on the killer's house . 
but thanks to poor editing , its had to believe neal could have murdered the killer anyway . 
bad cutting puts the timing way out . 
in other words , he's almost in two places at once . 
during this crucial scene we are also thrown a ridiculous red herring . 
by the film's end you realise that for the sake of an alibi neal must have self-inflicted a nasty injury in this earlier scene to make it look like he was attacked by a fleeing assailant . 
the injury ? 
he banged himself on the head with a bloody great rock! 
thus , the revelation of the " hero " as a killer ( and anti-hero ) in the final scenes may still prove shocking , but its all somewhat implausible by the time the end credits have rolled . 
helpfully , in the end we do learn that the bizarre flashbacks of personal humiliation and murder that have permeated the film belonged to neal all along . 
but there too is a problem , as the explanations offered for his motivations are psychologically fuzzy to say the least . 
the story owes us a better understanding of franciosa's mind , his concealed madness and his sudden murderous rampage . 
tenebrae is also marred by annoying little inconsistencies . 
for instance , a teenage boy and girl leave