I think it was his indifference to it, like jf you walked in on someone shocked or crying then okay you couldn’t have done it. But just standing over them with lifeless eyes is another thing…
to be fair, she did ask him if he did it and he didn’t answer and just had an ominous look so any sensible human being would assume he did it. keep in mind, she has spent her life witnessing his violent behavior so it’s not that far fetched
It’s weird that she assumes he killed him right away but aight