Just to say I think you are right to be concerned and I think your husband is unreasonable and it’s pretty poor behaviour on his part to say you are censoring or controlling him if you disagree with him.
Ash copilot or similar AI or even just google the link between exposure to violence and outcomes and you will find plenty of research on it finding extremely strong correlations between violent exposure and poor behavioural outcomes, especially in males.
Males exposed to very violent content (ie games or movies) when aged 3.5-4.5 showed a higher rate of antisocial personality and lower empathy.
Males exposed to violent content as teens showed 2 different negative patterns - higher rates of craving violent stimuli and desensitisation, or conversely a fear of the world and poor sleep. Girls didn’t show as much of the same pattern.
Once boys get to 20s it’s not as harmful as they are able to critically judge content and not have it embed so deeply in their psyche.
So it does matter in the same way that boys watching Andrew Tate or how to build bombs does matter. Plenty won’t be radicalised but these are formative years and some will be but even just some will be intrinsically shaped by this content even to a small degree.
Your son is watching this or reading it anyway so it’s a chance to discuss it and definitely a chance to talk about how poor childhoods and certain personality types lead to the creation of psychopaths and narcissists. But I don’t agree it’s an opportunity to discuss in detail the violence. You are right to feel this is inappropriate and I think your husband is sadly clearly one of those men exposed to ultra violence early on who now craves it and finds this normal.
If you find him the studies and show him the evidence it is actually factually proven harmful in boys would he listen? If no then you have a DH problem. I would personally stand my ground as sometimes when you are right - which you are - you do have to take a stand. I would also actively get your son involved in the psychology of these sorts of crimes but also try find good male role models to expose him to. Lots of true life crime stuff involves really tenacious male and female police or even parents and to me that’s a better way to learn about true crime and the world rather than the details of the torture which is let’s face it gratuitous.