I did once know someone to make a tongue-in-cheek argument that Mr. Del Mar took his sons to see Earl out of concern for their welfare and safety ("Be careful, boys, this is a homophobic society, so if you happen to be gay, be sure to be discrete!"). But unless you buy that, there's really no way around it -- Ennis' dad was evil. Ennis must have grown up in a state of contant terror.
Part of the rationalization I read was that this was the equivalent of shooting a dangerous animal in the neighborhood -- i.e., the Ennis' good ole dad had the best of motives in trying to protect his family from a man whom he thought was a threat to them, especially to his sons.
Horse puckey.
I live in an agricultural area -- beef cattle farming, to be exact -- and farmers around here have no compunction whatsoever about shooting a dog, a coyote, anything that's threatening their livestock. And I'm sure some of my neighbors have shot rabid dogs and other animals as well; and if an animal that can give you rabies isn't a deadly threat, I don't know what is.
However, the MO in these cases is you shoot the animal, kill it as quickly and cleanly as possible, get it over with. Anyone who went to great lengths to torture and terrorize and degrade the animal, show the mutilated body to their children and then laugh about it would be regarded by their neighbors, and quite rightly, as a dangerous sicko.
Lureen's father, I've heard suggestions that he might have had something to do with Jack's death. Don't see much basis for that, but I'm sure he would have felt, and expressed, some satisfaction in a circumstance where he didn't think he would be challenged.