ooh this is fascinating - as well as scary - I first had the idea that some sympathy might be required as it sounds like he might have got really worried about something being true and have added bits to it that don't fit in in time because of his fear and nervousness, but then when I re-read it it sounded like perhaps he was looking to you for an excuse as to why he'd beaten someone up, an excuse that you could give to the police, say, then I realised that I was being drawn in in the sameway you might have been - its sounds like you know he's wrong,m but I don't think arguing with him wil change his opinion of what happened - in fact it might entrench him. I feel for you here, as it must be very difficult - I do remember myself being in as relationship with a guy I never really trusted and my instinct was telling me that he was being deceptive and I would see signs etc that maybe weren't there.. but maybe were in another form- I know the powerful sense of wanting to scream 'you'r a liar' at him, and after an actual incident of infidelity, I almost couldn't trust anything he did afterwards. If he has no rational reason for feeling like that then you might have a problem - in my case, I would have needed constant re-assurance and clear positive signals for a llong time before I trusted again - so that would be my advice in that case.
HOWEVER i looked up the term 'Gaslighting' and realised that it was used a great deal on me by an overtly 'nice guy' boss, who was drastically incompetent and deceptive underneath (upwards they thought he was great, downwards we knew he was falisfying reports, not passing on information, then blaming people for not knowing it etc etc).
He and a colleague of his used to use the phrase 'you are misundertanding and misinterpreting what has been said to you' all the time -- in fact I think the colleague he drafted in had been given this script to deal with criticism as it was used with other people as well. If for example he'd said 'You are not pulling your weight, we need people to go the extra mile' and I then said 'well actually I already go the extra mile and help people out a lot' he would say 'you are misinterpreting what I have said' and I would say 'well what did you mean' he would say 'I said that I wanted you to stop giving too much time to helping colleagues you just said that you spend all your time helping colleagues' I would say 'no I said I already go the extra mile and help people out more than I need to, I don't think I spend too much time' and he would say 'you seem to be saying that you do spend time and you don't are you having trouble understanding what i am saying?
etc etc etc...
so I am going to say that even if its not intentional and he is having his own problems, it is going to take a lot of effort to hold on to your version if you and he are the only ones with an opinion on what happened - it might be better to take an 'he is ill' approach and try to coach him out of this paranoia - a friend of mine who had a miscarriage and felt very vulnerable (but totally denied the effects of the miscarriage) started reading everything her partner did negatively (eg if he was tired she accused him of 'sulking and ignoring me' if he went to work early it was 'because you want to avoid me' if he tried to talk it was 'you are interrogating me' so there could be some real emotional issue in the background that has triggered this as well.
I hope you work through it, but someone shouting 'you are a liar' is difficult - the only tip I can think of is to resist your natural defences and reactions (to shoult back, to re-state your story etc) and instead to say nothing or 'I understand I think what do you mean' until he starts explaining rather than shouting - often when people are shouting they are looking for a signal that you've heard (shouting is a way of trying top drive information in against resistance) and if you shout back or argue it will only escalate.
You might then find some clues to his paranoia and fear - perhaps about losing you, perhaps about feeling a need to take on anyone that threatens you?
Hope that ramble had some things in that might be useful if only to discard...