I had this crazy idea, lets just say it "came to me in a dream". What if someone who had been involved in the murder of SS directly or indirectly murdered PM & MM?. I will call Unknown 1 or U1.

U1 fears that his/hers involvement in the death of SS will be exposed by PM upcoming case. U1 has any number of reasons to be at Moselle, he/she is in with the fam. At some point U1 and/or U2 murder PM & MM, a victim of just being at the wrong place/time, leaving no witnesses. U2 could be a sibling of U1 or someone close to the M family, if not in it. Maybe U1 or U2 is already in some heap hot water & calculated that PM's upcoming case being heard could only add to his/hers issues w/ the legal system.