Even if you should learn that you do, in fact, have a human femur in hand doesn’t mean, ipso facto, that “This is from a crime.” Much less does it indicate “a murder”, on its own. To make a determination of “cause of death” will take the services of a medical examiner or someone with those qualifications.
There are certainly ways that an animal could come across a human femur – an entire body, in fact – which don’t mean “this is a murder victim”.
Homeless people die of exposure frequently enough, or people in accidents while hiking rough terrain, and are sometimes not discovered until years after the fact. It happens. Sometimes abandoned and/or forgotten graves are even exposed through natural events, such as flood and erosion, which can mean that a previously lawfully interred body can be uncovered.
Even suicide can’t be ruled out, especially if the victim had somehow managed to do that in a place where the body might lie undiscovered for a long enough time for the flesh to decay.
And I don’t profess any expertise in human morphology, but that bone still looks too big for less than a giant of a person. Got any beanstalks nearby?