pima county sheriffs say david watson , 46 , killed his ex linda watson while they fought over custody of their daughter in 2000 .
three years later , police say watson murdered his ex mother-in-law in a drive-by shooting at the very home where linda watson first vanished .
marilyn cox had been fighting for visitation rights over her granddaughter - she was gunned down alongside her neighbor renee farnsworth in 2003 .