On a second re-watch even more questions.....(SPOILERS)
In watching Reptile again, (which I do suggest because it allows a viewer to see different things and to focus on things that deserve more focus but may have been overlooked on the initial viewing), other questions have emerged.
For one, I'm assuming that Will (Timberlake) and Wally (Lombardozzi) killed Eli (Pitt) and disposed of the body, and believe this was indicated by Del Toro (Nichols) finding the empty bottle of bleach and no Eli. Though, I'm surprised this plot point was then sort of dropped.
Next, I do not think Will killed Summer. I think that was Wally and this is because, recall, Will was with the other man signing closing papers, and not only that, he had no blood on his clothes at all and her body was drenched with blood. This could have just been a mistake by the makers because he did admit to giving Summer CPR (and his clothes still did not have blood on them). But also, recall, someone unknown entered the house while Will was with that other man. I think Will came back and found her still alive and either waited for her to die or maybe helped her demise.
A second viewing also allows so many other tiny subtleties to be caught - for example, Nichols removing his earring on the way to the murder scene, which I took to symbolize that recreation time is over and replaced with work time (but then later, the earring is left in).
Then, all of the sideways glances, glares, pauses, and acting nuances that become more clear.
Anyway, I do think this is one of those movies that grows on additional viewing and I wouldn't be surprised to learn that the director is one of those people that has a dislike for making everything in a film obvious.