There is an omitted e at the end of the modal verb:
würd → würde
And there is an omitted subject. You can see, that the verb is conjugated in first person, so the subject must be ich (I):
würd → würde → ich würde
Now we have (ignore the question mark at the end for a moment):
Ich würde ihm gern das Fell gerben dafür, dass er vor den Leichenfressern weggelaufen ist.
And this is in English:
I'd like to give him a good thrashing for running away from the corpse-eaters.
(»Jemandem das Fell gerben« is literally "to tan somebodies coat/fur" but it means "to beat someone really hard in order to punish him")
So, I think the funktion of the subordinate conjunction dass is clear: The pronominal adverb dafür (for in english) needs a subordinate clause that explains for that he should be thrashed, and in German this subordinate clause must start with the subordinate conjunction dass.
So, what about the question mark?
This is a statement:
Du isst. (You eat.)
And this is a question:
Isst du? (Do you eat?)
But what is this?
Du isst? (You eat?)
The word order says: This is a statement. But the question mark says: This is a question. The truth is: It is both. You use this construction to express surprise.
Your son has told you he is eating vegan since some weeks. Some day you enter his room and catch him eating a hamburger. You are surprised and say:
You eat a burger?
This is both: A statement (from the word order) and a question (spoken from the intonation and written from the question mark).
And also this sentence is both:
Ich würde ihm gern das Fell gerben dafür, dass er vor den Leichenfressern weggelaufen ist?
It is a statement: I want to beat him for running away.
But it also is a question: Do I really want this? Does he really deserve to be punished?