It would have to be the science. And that is tentative until I read more documents from both sides (by science I don't mean only Stefanoni).
It is not their demeanor, their quirkiness, their looks, their family, their friends, their extracurricular activities, their sexual activities and so on. And as much as I don't think the characterization of Mignini and Stefanoni is fair by many innocents I also don't think the characterization of Amanda and Raffaele by many guiltys is fair either.
I appreciate that you have not been swayed by the personal behavioral issues. So you (at least currently) believe that the DNA results are valid? I can understand that position, if only because I have personally been skeptical when defense attorneys claim the police botched the DNA process in other cases.
Here is what I see as the prosecution's case:
1) Assumption that Meredith had to have been killed by more than one person, even though 7 out of 8 experts at trial did not support that view
2) Meredith's DNA on the knife blade (although there was no blood or human tissue found, and Stefanoni's testing, which others disagree with, cannot be replicated)
3) Raffaele's DNA on the bra clasp (issues have been fully discussed here)
4) Curatolo said he saw Amanda and Raff in the square (he was admittedly high on heroin, and his testimony does not fit the narrative, and makes it impossible for them to have done it if true)
5) Nara heard a scream and footsteps (but doesn't know when, and didn't see anyone)
6) Assumption that bathmat print is Raffaele's, although expert testimony did not prove this
7) Amanda's DNA in her own bathroom
8) Luminol footprints, some with Amanda and Meredith's DNA both, that tested negative for blood, and are in the home where they both lived (and not in the room where the murder took place)
9) Amanda's "confession/accusation". Except that it doesn't fit any of the other facts of the case, and was clearly coerced.
10) Assumption that the break in was staged (also not supported by the evidence, IMO)
I will agree that IF the knife DNA is valid, there
might be a case, but the prosecution would
still have to figure out how and why the two decided to kill Meredith in conjunction with a third guy they barely knew and did not hang out with (and did not have any communication with). And come up with some logical narrative, not a fantasy. And come up with a valid motive (I suppose if the evidence really clearly pointed to the pair, we could go with the "we just don't know" theory, but only "if").
To me, there is no case at all. We all would have our own DNA in our own homes and bathrooms. Even if we killed someone, that would not prove anything. And how did Amanda participate in the killing and not leave any trace in the murder room? Yes, that happens, but not in brutal, messy cases like this.
OK, done. I can see how you could be swayed by science, but not when it doesn't support the overall story, and not when it was done as shoddily as it was here. Bad science + doesn't match the other evidence = invalid results.