I'm going to take a type out loud moment here and say its a deep self rejection-projection. This is based on years of experience, both as a DA thru and thru, and being married to one (not sure new to the AT stuff but he is maybe DA or FA/with DA tendencies imo, and not open to discussion so going off of how it looks from outside) as well as primarily dating and being drawn to DA relationships (romantic) most of my adult life ---contrary to what the alot of the books suggest just doesn't happen. Idk how much I can relay about this here but basically my SO turned on me and became very mean a few years into marriage, before this I could literally do no wrong, he had a long history of being verbally abused by his dad, called every name worthless/no good/moron etc.... his mom warned me how before marriage she could do no wrong and after she could do no right with his dad and with my SO it was pretty much the same, took a few years but yes it became truth. " The harshness, the brutality of his tone" as you wrote, I can so so so relate its so awful, it basically killed what we had.
In hindsight I can see as a self sufficient full fledged DA I didn't need anything from him until we had a child and then as we both decided I'd be a SAHM suddenly, looking at it thru the AT framework lens, I appeared needy then (to him not me in reality, as a DA I had my own savings, had paid my home and cars off with my job etc I felt very good about the facts of where I had placed our financial sit as far as that went) and also he saw I was trapped with him, he had me and knew I'd not leave him easily then, and he was right, but wow the mean-ness was heart breaking I can't even tell you the level of unkindness and cruelty made worse from the fact I was falling off a tall platform/pedestal I never asked nor wanted to be placed on so the difference was like a nightmare. Anyway in hindsight it also caused my own DA to be in full force and I totally closed myself off from my SO and have not changed from that position. Deactivated is a very big understatement, I have to keep pretty much everything from him he is not open to letting me have therapy or meds for anxiety or doing activities basically anything I enjoy that was fine before then is now not allowed. So I do what I want and keep secrets which fits my DA self just fine. I tell him now I am mean to him as a self protection, that he taught me thats how he needs treated for him to treat me in a respectful way (sad but true I switched the script on that) and I also tell him if I show him any iota of love he goes into full cruelty mode (he adamantly denies this but.... facts), he cant show me love unless he believes I have a foot out the door as well (then he love bombs which is just so gross), so its a pretty horrible cycle with all kinds of other dysfunction in the mix--- bc we are not our AT styles we are more than that but it sure does explain alot and can predict to a T how reactions to certain things will play out. For myself I can say I'm trying to get myself out of this cycle and away from this and fully detox myself.... get back to my real self .... (literally my dream is to be alone and detox and be alone... and detox.... alone haha.....being disappointed in myself for not leaving and tolerating it, I can relate too also but its complicated, getting there.) ....getting back the real me who is a kind and loving person when not being treated horribly. I am very kind to everyone until they are mean to me, betray me, slight me or judge me less than, then all gloves come off and yes I can cut to the quick then with no remorse this was even before my current SO so he had no clue what he was awakening when he went from kind/loving to cruel and disrespectful, the DA self protection mechanism kicked in and has not turned off in our relationship.
So TL;DR mean-ness as a DA-- parts of self protection (this isn't saying you deserved any meanness but his DA self may have been triggered by who knows what so self protection kicked in with DA critical eye for everything).... as well as self projection (ie self hate), speaking only for myself here but I know deep down I'm not a big fan and I'm working on that. Hope this helps someone!