One of my first thoughts about Durgy was that he should have been investigated further, although I don't think he's involved. I also wonder if LE asked the counselor he was seeing what was discussed. Why could Durgy no longer not face his students? (He said this before Betsy's murder.) It appears that Durgy took his own life, but why? Did his wife have any infor to offer? Unanswered questions, for sure. Finally, he was not identified as being in the library by anyone.
Actually, Durgy was investigated rather lengthily by police both from Michigan and PSU. I think part of the reason he may have committed suicide was that in his unbalanced state, to be investigated for murder may have pushed him completely over the edge.
There are a lot of reasons why one might not be able to face their students. Having been a teacher myself in a past life (not literally) I can attest to that.
Especially with the war going on -- he may have felt as though teaching English was not effective at changing the world, he may have worried about what would happen to students who failed out of his class (drafting, etc.) or he may have just gotten culture shock moving his family to PSU (basically, a backwater) from Ann Arbor.
I would even suspect perhaps he just became depressed from moving away from family and friends. I know a co-worker of my wife right now whose husband is very badly depressed because they made a move from Baltimore, PA to the local area, and he feels as though he is "too far" from his grown children, the places he used to fish, etc., so he basically mopes about and wishes to die.
I know that his wife confirmed that they were not in State College that day, having left the previous night. They missed Thanksgiving dinner as she remembers it. Other professors also noted his absence.
Plus, that would not explain the further contacts from the murderer -- the postcard, the shrine(s), etc. which occurred after his death.
He is interesting on the surface, but once you dig a little deeper, he becomes a less compelling suspect, and more of a sad case of a guy who has just reached the end of his rope, for whatever reason.