>>262 "Human men are rougher than I realized." doesn't sound like good English to me.
人間の雄は想像以上に下劣ね, I would translate as "The human male is vile beyond imagination, isn't it?" or "It seems that human males are beyond sordid." or "It's unbelievable how nasty those male humans are."

>Isn't "than I expected" commoner or more natural than "than I realized"?

Different meanings. Besides, none of these are in the original sentence.

>Why is the "Human men" plural?

"The human male" can mean every human male, or it can mean a single human male. "Human males" can mean every human male, or it can mean a group of human males.

>Doesn't the meaning of "human" and "men" overlap?

Human is the race. Men can be the race, or it can be the males only. That's why it's a bad idea to put "human men" together, it's confusing.

>If I write "Human males" instead of it, what do you feel about it?

Much better.