Just eye-witness testimony. Unfortunately for your fantasy scenario, that's enough.
No marks on the grandfather? Attacks and punches to an eighty year old grandfather didn’t leave a red mark or bruise? No swelling or redness reported by a person on heart medication?
The grandfather followed the father out of the house and killed him in the driveway?
Yeah, unbiased third party testimony from the granddaughter and other family members of the shooter./s