You really only need to hang mean bastards. But mean bastards, you need to hang
I think it's a question for those who've seen the Hateful Eight movie because I don't know how to describe the context here.
If I heard it right John Ruth in the first part of the sentence stressed the "hang" word".
So, my guess, it goes like this:
You only need to HANG mean bastards, but MEAN BASTARDS, you need to hang.
The problem is it doesn't make any sense to me.

Question

You really only need to hang mean bastards. But mean bastards, you need to hang
I think it's a question for those who've seen the Hateful Eight movie because I don't know how to describe the context here. 
If I heard it right John Ruth in the first part of the sentence stressed the "hang" word".
So, my guess, it goes like this:
You only need to HANG mean bastards, but MEAN BASTARDS, you need to hang.
The problem is it doesn't make any sense to me.

Answer

I have not seen this movie, so I could be wrong, but Jon's answer doesn't make any sense to me.  Without knowing the context, I would analyze the sentence this way:

"you only need to HANG mean bastards" = There is only one kind of person that you need to hang. (You do not need to hang anybody else. You "only" need to hang "mean bastards.")

"but MEAN BASTARDS, you need to hang" = You definitely do need to hang mean bastards. (Or: When it comes to the subject of "mean bastards," you have an obligation to hang them.)

In other words: "You ONLY need to hang mean bastards (not anyone else).  BUT you definitely do NEED to hang mean bastards (you can't just avoid hanging anybody at all)."

That's what it looks like to me, although again, I have not seen the movie.

Answer

The only people that need to be hung are mean bastards. All mean bastards should be hung.

Answer

What is being said, is that "mean bastards" need to do the hanging and they need to hang, "mean bastards."