However, there is no one else in his life who's getting this much detailed information, so there's probably no one else to try to get him to see the other side — and that he might be mistreating his co-workers. So, what do I do?
— Conditional Ally?
Conditional Ally?: Tough one, because being in the “supportive listener” role for someone who is oblivious to their own culpability is a contradiction in terms. Your silence isn’t supportive, it’s making things worse.
But as you’ve seen, speaking up in a way that contradicts his victim narrative is not something he welcomes.
One workaround is to wait until he’s not unloading on you, and ask to speak generally about the way you and he talk about outside conflicts. Present this hypothetical: He’s venting, you’re listening. You notice he might be misreading a situation or unwittingly making something worse. Would he rather you just nod and listen, knowing it means he might go back and keep making things worse — or would he rather you point out the thing you noticed? If the latter, is there a way you can say it that won’t set him on the defensive?
Offer the other side by saying how you hope he will talk to you when you unload something on him.
Even if he so lacks self-awareness that he gets defensive anyway, it’s something you can cite calmly: “I thought we’d agreed on this. I’ll drop it.” The whole conversation, firmly but without rancor.
All confidants could benefit from such a conversation, a clarifying of terms so you can support each other effectively. It also (maybe a bit late in some cases) can help you get to know each other better and even see how well suited you are to each other, since a “tell me straight even if it ticks me off” person might struggle to get along with an “I expect you to lie to me because you’re my friend” person.
If your husband stiff-arms all unwelcome truths, then suggest — again, at a later time — counseling or conflict-resolution training to ease his work stresses. It’s one thing to have the occasional rough week; it’s another to bring some sort of drama home every day.
Maybe solo therapy for you, too, on life with Mr. Can’t Be Wrong.
RE: Conditional Ally: Oh, my. That was me 40 years ago. My then-husband was a perpetual victim of his business partners, and could not accept that their actions could have had a different context — e.g., not just for the purpose of screwing him. He eventually decided I was also "against him," and we divorced. Thanks be to God.