who killed Summer?
It's never said. One of this movie's many "mysteries"1. Reptile it is a film – by narrative choice – full of details and clues which, combined, (should?) lead to the culprit. But its plot adds too many subplots, making the big picture blurry.
how is the witness's description of the killer having a limp relevant?
Many murderers, especially in this era of CCTV, try and hide at the best of their abilities who they might be: hoodies, gloves, glasses, wigs, disabilities and so on... It seems logical that whoever did it, they tried and hid themselves, leading the police on a wrong track. Any potential witness may describe the killer as limping (think "Keyser Söze" here).
Like many questions one might have about that movie (I think of this one too), this one might not have an answer. Some people like that movie and directing style, some (like me), don't. Because, as Hugo Prevost writes it in Pieuvre:
Reptile est donc un drame qui tient bien la route pendant les deux tiers de l’oeuvre, avant de s’essouffler et de se terminer sur un pétard mouillé. Dans le contexte, qualifier le tout de « David Fincher léger » semble approprié. N’est pas Seven qui veut, après tout.
Translation: Reptile is a solid well-constructed drama for the two-thirds of the movie, before slowing down to the point of ending like a wet firecracker. Calling it a "David Fincher light" (read: zero / of the poors) seems appropriate. Not everybody can make "Seven" after all.
Roger Ebert is even more caustic and biting.2 But Netflix explains some of the inconsistencies/twists as part of the plot.3 The writers/filmmaker/producers maybe wanted to show "the good and the bad" as one person, when, like reptiles, shedding skin would symbolize a re-birth, and make the bad become good. But they left too many aspects of the story and the plot unexplained.
1. TIME says it might be "a loner with a Charles Manson-like stare (Michael Carmen Pitt) [because he] is seen slinking around the crime scene and elsewhere—maybe he did it." But it's also said that "*This is why Will killed her – not because of the affair, but because she threatened to expose his scam."
2. Benicio Del Toro slinks and weaves through Grant Singer’s debut thriller “Reptile,” but the film struggles to develop a confident personality around him, ultimately coming apart at the seams. Clearly inspired by David Fincher’s meticulousness—Singer too is a music video vet and has worked with The Weeknd, Skrillex, Sam Smith, and many more — “Reptile” is overly mannered and precious with its details, but its biggest misstep is its failure to understand that procedurals need to get narratively tighter and not just more convoluted. Del Toro always brings it, and this is actually one of his more intriguing performances in a long time, but one consistently wishes that it was in a movie that knew what to do with it.
2. > Why is the movie called Reptile?
> “In the movie, characters are introduced as one thing and revealed to be something else,” Singer continues. “There’s a shedding of skin that occurs, and it felt like an appropriate metaphor for the film. Some of the most unethical people in the film can be very likable in moments.” Like anything with cold blood, these reptiles can be perfectly warm under the right circumstances. In a film full of mystery, even the title is a bit ambiguous.