Out of all of the possible scenarios, to me, accident sounds most believable. In the accident scenario, only a fun prank was planned, there was no malice planned. There was no tension between Meredith and Amanda. There was no planning by Rudy to rape Amanda or Meredith or anyone. There was no planning to stab someone. There was no planning to kill someone.
It makes the most sense. They did not mean for it to happen.
Meredith was accidentally very badly wounded b/c of the prank. She could identify all and would get all in trouble. They panicked, the wound was very bad and there was blood starting to get everywhere. They then made a series of terrible decisions, because they panicked and maybe because they were young-ish. Raffaele worried about what his family would say, how would he explain this, what would happen to him? Amanda also worried about how she would explain this to her family, also this meant her whole dream of study abroad and all hopes and excitement would be ruined, her fresh start with Raffaele would be destroyed. Rudy worried about being put in jail.
So they decided they couldn't call for help and have to explain this. But what were they going to do? Meredith was already dying in front of them. They decide that they have to make it look like someone else killed her....like someone random came in and raped and killed her. They had to stab her more to make it look like it was a murder. They had to stage her body to make it look like someone raped her and murdered her.
They had to stage the window so it seemed like someone random came in from outside.
This scenario, IMO, explains a lot of things. They were not evil. An accident happened, and they panicked and made terrible decisions.
And, I might add, it would also account for Amanda and Raffaele's actions afterwards, in the years after the murder. Yes, Amanda is hurt by how the media portrayed her - she did not murder Meredith out of some kind of hate or some kind of evil-ness. How could people say things like that about her? She never meant to hurt Meredith. Meredith was a good friend, they were very good friends. All of that would still be somewhat true from the above scenario. It would explain how both she and Raffaele can go about writing books and doing all these sorts of things. Because they are not evil killers, and they are not who the media is making them to be. And perhaps others can learn from their "story." I can see how they could end up seeing themselves as the "victims," I can see it very clearly. How the above scenario and its aftermath could end up making them see themselves as the victims, just as much victim as Meredith. It makes some sense from that perspective.
It makes a lot more sense than them killing Meredith as a result of things getting "out of hand" or some kind of argument gone bad. The actions of Amanda and Raffaele line up much more with the accident scenario, then with this scenario. In that case, they would just be really really cruel and how could they ever see themseves as victims? And do all of the press and the books, etc.? There would have to be something very wrong with the both of them.
IMO, accident scenario fits in very much with both the innocent arguments and the guilty arguments.