First Scene is a Plot Hole

They all agree to leave, set up their alibis, and then he comes back and calls everyone "acting" shocked so they can set Vince up.

General timeline:

Luke kills girl (he thinks), calls everyone over. They come up with plan to frame Vincent for suicide. Everyone leaves and establishes alibi while Philip stages the scene. Philip decides to slit her wrists (actually killing her) and leaves.

Luke comes back to start the 'fake' finding of Sarah, but instead of coming back to a pill suicide, he comes back to a blood bath (ish) scene, which caught him by surprised (I would assume) since they seemed genuinely pissed that Philip did it that way.

Over-reacting? Yes, he thought she was dead already, he already had a plan about dealing with it, so realistically maybe dropping the bags of groceries makes sense, but the acting afterwards (tears etc.) was over the top given what we know about the scenario.... even if he did have feelings about the girl strong enough to kill her.

No you are wrong. He was not surprised that she was murdered. He was shocked at the gruesome way she was killed, which was not discussed prior. Hence, the dropped bags.

~ Well, if this is it, old boy, I hope you don't mind if I go out speaking the King's ~

In the begging you see the killer with groceries going to the loft (not sure why he goes back out to buy groceries and start acting shocked when he sees her if hes on his own).

Anyway, the point is, when he sees her (in the beginning of the movie), theirs blood all over the bed with the Spanish writing with her blood. But that doesnt happen until later, that happens after he calls his buddies and they agree to frame the other guy.

The whole movie was set up for a plot twist that didnt add up.

I think you missed some parts of the movie.

Luke saw the girl get broken up with on the video, decides to go up there after Karl Urban's character leaves. Drugs her, and makes it look like a suicide. He calls everyone besides Karl Urban's character, and pins them all against him so they would want to set him up. They all leave to go about their alibies so they are covered.

Luke then comes back with groceries (my guess is he brought these to use it as an excuse to go there), when he gets there he drops the bag because he did not think Phillip was going to go as crazy with it. He says that later on in the movie as do Chris & Marty

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