It's an academical point right now, given your later post, but even “X participated in killing of Kercher” is not entirely clear. Any action A, from cutting her throat, to holding her, to opening the door, to simply being in the room, can be considered as “participating” in the murder, in the sense that suitably complicated scenarios can be constructed where X not doing the action A would have avoided the murder, with X knowing it and deliberately choosing to do A in order to cause Kercher not to live anymore.

I imagine what you actually wanted to ask was something like the probability that “X is guilty of whatever X was convicted”, though that only moves the uncertainty about what you ask to uncertainty about what Italian law says.