We don't know anything about Alma's parents, so I think it's too rash and risky to say that Alma's father would have killed Ennis.
Not everyone back in the sixties was a killer. To think that is crazy and unfair. There are so many husbands being unfaithful to her wives. Their fathers obviously are mad but they don't kill their sons in law. Someone has to be a really bad person with very cold blood to be a killer. I don't think every father with a son in law unfaithful is a killer.
We don't know even if her father could be homophobic. Not everybody was like that back then. But if even he was homophobic that doesn't mean he was a killer. To think that is being unfair with the human beings, thinking that when anyone doesn't like something or someone the only human reaction is to kill. The reality is not like that. In that age, before, after, there have been gays and a lot of homophobic people have known that, but not everyone who didn't like that kind of people did something like beat or even kill. To be homophobic is one thing, a killer is other. There are so many things or people we don't like but we don't kill them, right? How many people has a friend or a relative suffering because of a unfaithfulness? And are they killing those unfaithful people?
Maybe Alma's parents were like Jack's parents. Religious people, unable to kill. Look at the old Twist. He hated his son and he knew the truth about his son, but he didn't kill him. And what about Aguirre? He thought that he has lost so many sheep because of Ennis and Jack f***ing instead of herding. He was sooooooo mad, and he was a homophobic man, but he didn't kill them, and he didn't tell someone to kill them. Even we said once that Ennis was homophobic but unable himself to kill someone. So maybe Alma's father could say horrible things to Ennis or maybe could beat him, but kill him? It's to judge someone without proofs. We didn't know him. Not everyone is a bad person, not here, not there, not always.
Not all the racist people kill, right? So not all homophobic people kill.
Where is our faith in the human being?