It is fairly obvious that Guede killed Kercher.
What seems equally obvious is that Knox and Sollecito had a pretty good idea that Guede was responsible (and probably believed his likely claims that it was an accident), but concealed their knowledge from the police because telling the unvarnished truth would have revealed that they were drug users and possibly low-level dealers.
This would make them accessories after the fact.
As to your point about Italian law, double jeopardy is unconstitutional here but is also a principle that deserves universal approbation - without it, every person is vulnerable to vindictive prosecution.
This case has exposed a flaw in the Italian legal system.
She may have been a scared, ignorant, pot smoking skank, but that does not make her a murderer. If anything, this case exposes how out of control the Italian prosecutors are; they should have their credentials revoked.