I can’t exactly claim to be qualified to diagnose anybody, although I have taken some psychology courses at my university and have been reading a lot on the subjects of psychology-psychotherapy-psychiatry in my free time for years... and yet.
Remember Johnny saying this:
Johnny: (...) Boyd doesn't give a damn about nothing. Never did. When we was kids, he'd build half a tree house, and then he'd run off and go build a go-kart. And then he'd forget them both and spend the whole rest of the summer stealing candy and trading it for cigarettes. (...)
And then Bo saying this in 1.11:
Bo: I seem to recall you making your first tent out of bedsheets and broom handles. You were about knee-high. Some things never change, huh?
Because Bo has never been there for Boyd, building up his crime empire and-or prioritizing Bowman over him.
And that other time in 1.11:
Art: Well, it's now the property of the Marshals Service, until such time as you can prove that you came by it honestly, you know, with donation forms and receipts. And I'm sure you've got all that stuff.
Boyd: Well, I've never been good at paperwork, but I'll get right on that.
And now think about how Boyd has changed his, hm, occupations multiple times over the course of a couple of years. And then how he blew up Bo’s shipment in 1.12, betraying Johnny, as Johnny explained to him in 1.13:
Johnny: You have any idea how ugly it's gonna get now?
Boyd: I am ready to reap the whirlwind.
Johnny: God! Well, I am not, okay?
And that time when Boyd came to then already incapacitated Johnny in 2.10 to share with him his thoughts and feelings (apart from inviting him to go back into business with him), and Johnny explained to him once again how inappropriate/egotistical his behavior is:
Boyd: It's not just about me, Johnny.
Johnny: It's always about you, Boyd.
What I’m getting at is, I think that (apart from PTSD from the war) Boyd might have some form of ADHD. I mean, we’ve got recklessness (remember the robbery in 1.1, that’s all I have to say about that, haha), a lot of self-focused behavior, lack of organizational skills at times (which is why Devil and Johnny were so helpful to him), unfinished tasks, daydreaming, etc.
I feel like this hypothesis is worth considering.