So whodunit? There’s a ridiculously complicated answer to that question!
First of all, the victim (face-down and not immediately identified) is Sarah, whom Vincent has been seeing on the regular. Vincent has also been sleeping with a number of other women: Marty’s wife, the virginal sister of Philip’s new wife, and Anne, who turns out to be a prostitute he’s hired to seduce Chris. (I’ve seen both this version and the Belgian version, and I still can’t recall exactly why he makes this latter arrangement.) How do we know Vincent has been this sexually active/diabolical? Because Luke, giant creep that he is, has been videotaping everyone’s lovemaking sessions for his own sick edification. He’s especially obsessed with Sarah—whom he likes, too, but who rejects him coldly.
As for what really happened to Sarah, that’s hard to explain but I’ll try. After Vincent called off their relationship, leaving Sarah a crying mess in the loft, Luke comes swooping in to console her. Or so she thinks. Luke has actually drugged Sarah’s wine, and once she passes out cold, he injects her with a (seemingly) lethal dose of insulin, writes a suicide note, and leaves her splayed out on the bed with a bunch of sleeping pills scattered around her.
But wait, you might ask, what about the handcuffs? And the blood? Okay. As it happens, that was a bit of stagecraft arranged by Luke, Chris, Marty, and Philip to get revenge on Vincent for betraying all of them. (Luke has shown the other three video of Vincent’s affairs.) So the guys drug Vincent and handcuff him to Sarah. Needing to clear the room of all traces of their presence there, Philip volunteers for the job and shoos the other guys away. But he’s so overcome with anger that he slashes Sarah’s wrists, thus setting up Vincent for murder. We learn later that Sarah actually survived the drugging, but was actually killed by Philip, who did not intend her harm.
And now that leaves creepy Luke, whom Chris unmasks as the man responsible. The two get in a tussle on the balcony. Once Chris gains the advantage, Luke concedes and leaps to his death several stories below. There are postscripts for all the survivors, but they’re too dull to sort out.