Once again, you reverse the burden of proof. Fortunately this was something that the prosecution DID try to prove at trial, and they failed.
You now wish to say that this failure to prove the "bad relationship is the motive" theory was not tested and found wanting. Some of the British girls testified, Filomena testified and the triers of fact came away saying, "there is nothing here which would count towards a motive."
The prosecutors did not change their allegation of motive. They went to trial with it, after their "sex game gone wrong" motive failed, and after their "ritualistic satanic theory" failed - that one failed so much that after talking about it for a year they did not take it to trial.
At some point you're going to have to admit that all of this was alleged, long ago, and when tested in court it failed. The best Massi could come up with - a theory that not even Mignini advanced - was that the pair made a momentary "choice for evil", which even Massei admitted was against their characters.
Read the Massei report. Based on the Massei report, we actually CAN argue that these two girls were in the midst of a normal friendship, with normal household tensions. None of that was found by ANY court to have cotributed to the murder.
Yet here we have it, 5 1/2 years later, people saying that it does. You'd think this had never been to trial.