At the very least, it's highly improbable (and in my view impossible) to prove from direct observation that all slave owners were cruel, since we don't have recorded observations of every plantation or household that "owned" slaves. In theory, there might be an occasional plantation that treated slaves as well as hired laborers were treated (except--as you've pointed out elsewhere--that slaves were punished for leaving even these theoretically benign households).

However, it's easy enough to infer not only that all slave owners were cruel but that cruelty was endemic to states that had fought to preserve slavery: it's clear from practices, codes, public lynchings, laws, and other forms of punishment that arose during Reconstruction that as conditions of cruelty and servitude were forced on black people, practically no white people did anything significant to ensure Constitutional equality for all. The myth of "benign slaveowners" went hand in hand with the myth of "separate but equal."