If your BIL did indeed do this then if I were you I would be sleeping with a house alarm set,atleast 2 big dogs,and a gun. He sounds like a nut job and I am concerened for your own safety.People kill their loved ones,a neighbor,or a stranger if the trigger sets them off. Take what steps you can to make yourself safe. Lock,game cam,and maybe a leg trap outside the pen!
Sorry for your losses and the possible cause. Hope you and the birds will be safe.
Will you or anyone be asking BIL about this,or will he crack and tell like he did about the fence? If he does tell someone needs to get him into mental treatment.Inpatient to start off with. My dh doesn't like my hens all that much,but to kill them is not something I can pciture. I just can't see how BIL could do that without being mentally ill.