What is your personal opinion of military members who desert and then want to badmouth the military?

Not sure about the Brothers, but this Sister believes as you do. All Volunteer means its a singular decision to join or not. Its been my experience that those who go AWOL are often being considered for punishment of some sort, or they have this mental view that they are being singled out for some reason or other.
While in, I used to drive the Brig Runs, taking SM from Cherry Point, to the Brig at Camp Lejeune, and they are treated as Brothers or Sisters during that ride, so conversation with the Guards was common, and though I didn't take part in most of those talks, I did listen. There were all sorts of "...THEY singled me out for punishment that I didn't deserve.." and the "...Stupid Regulations.." idea, but the one that always stunned me was, "...I was already in trouble, so what did I have to loose?"
Enlistment is much like a Marriage in some sense... if you can't deal with the restrictions and the expected conduct, then don't do it, because you can screw up your entire life if you mess it up.

My opinion of a deserter?

There is no one below him/her on the food chain, and I don't give a *&^%* what he/she has to say.

The only opinion I will listen too from such is whether they want a blindfold or not at the firing squad.

It really all depends on the condition of his enlistment. I know you said in-law, but that doesn't really list age. Let's not forget that while we are an all volunteer fighting force, that hasn't always been the case. I do not look kindly on deserters, but draft dodgers is a different story. I might not respect them, but at the end of the day, they didn't sign up for what we did.

But if we're talking about a recent desertion, that might be a different story. Especially if he's constantly badmouthing the military, I would remind him that one phone call could land him in cuffs at Leavenworth. While they may or may not be true, it might shut him up around you so you don't have to deal with such blatant disrespect.

I can pretty much shut out when people badmouth the military, but when it comes from family I'm going to get heated.