I think motive, especially in this case, goes beyond did so-and-so have a reason to kill. It's their motivation to engage in a plausible narrative. AK and RS at 8:40 learn their evening is free. Within 20 minutes they grab their biggest chef's knife to kill Meredith with in Amanda's own house, hope she's actually home, then recruit the local burglar on the way for no reason, and he agrees to tag along and rape the corpse for good measure. These actions are a sequence of social non sequiturs compounding on one another. It's evidence it didn't happen. Could it override AK's blood prints on the knife, no, but lots of different types of credible and probative evidence could be weighed against each other in such a manner.
To me, this is a good summary of the role of "motive" in the AK - RS case. There was no credible motive ever expressed to tie the defendants, Knox and Sollecito, to the murder, and none to tie them into a collaboration (conspiracy) with Guede, the convicted murderer. The lack of credible motive and the lack of credible evidence of Knox or Sollecito being present in the murder room both support their actual innocence.
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