Sometimes some of the questions on IMDB amaze me. Do people just skim through movies before asking the questions?

Will had no reason to kill his wife. He had already moved on with another woman and it was not in his nature to murder. Stewart was the only character who had a motive. He wanted to frame Will.

Yes, it can pretty much be proven. Stewart basically admits to it when he first captures Laura. He says something to the effect of, "Don't play games with me, you know what happened to Charlotte." In other words, he meant "Don't piss me off or I will kill you like Charlotte."
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It was definitely Stewart who killed Will's wife. He's not in the slightest bit surprised when he hears about the murder on the news. In fact, his first reaction is to make sure that - should Will be proven guilty of the murder - he gets the job he's been chasing since the start of the movie. It's his sole motivation from beginning to end.
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interesting.
His namewas Julio Iglesias!

What?
Stewart kills Caroline and attempts to frame Will for it in order to get his job as editor-in-chief. The reason the samples are contaminated with canine dna is that Stewart is turning into a wolf. Of course we don't find this out until after we learn of Caroline's murder and the contaminated sample, meaning at first we suspect Will might have done it.
Reality is the new fiction they say, truth is truer these days, truth is man-made

i thought will did it & the cop made up the excuse of evidence being of the animal or whatnot b/c of who he was dating, he found out.
His namewas Julio Iglesias!

I don't have the forensic evidence if that's what you're asking. But it's implied.
Reality is the new fiction they say, truth is truer these days, truth is man-made

could that be proven?
His namewas Julio Iglesias!

Nah t'was Stewart.
Reality is the new fiction they say, truth is truer these days, truth is man-made