I understand why you're upset. When someone had or has views that a) I personally find abhorrent or offensive and b) nowadays most people would find abhorrent or offensive, it is very hard to listen to a third party speak of them as a rounded person with good points too or to listen to abstract musings on why that person holds the views they do or listen to someone put those views in a context.
It's hard because it does sound like a defence of that person (which it often is) and, frankly, it's easier and more satisfying for me to despise and dismiss that person wholesale. I also want people to unreservedly take my side and wholeheartedly condemn the other person, dammit!
I think that's completely natural, and I would never beat myself up for feeling that way, but it's the gut reaction, not really the grown-up or helpful line to actually take once you've thought about it.
This person is presumably your MIL's beloved father or FIL. She knows full well that there were wonderful things about him as a person when he was alive, she is correct that his views would probably not have been unusual at the time (and let's not kid ourselves that we all carefully and consciously form our own views in every topic, including those that never really affect our lives - we largely pick them up from our social groups until we have real cause to examine them), and in any event he is dead and she probably has no desire to speak ill of him given that.
You're not unreasonable to think badly of him - you're under no obligation to give him the benefit of any doubt - but you were and are being unreasonable to expect your husband family to speak ill of their own dead relatives to prove their own views on a particular topic or their allegiance to you in some way. You were also unreasonable to basically shout at them because of it.
To be honest, I think you'd understand that a bit more and be willing to let it all lie - knowing that this was a difficult situation for them as well as you - if you otherwise got on, loved them and knew they loved you.
Ultimately, I think you have to ask yourself what you're hoping to achieve here. If you're waiting for them to wholeheartedly condemn their dead father/grandfather to please a woman who doesn't like them anyway, I suspect you're going to have a long wait.
You've apologised for your shouty email. Sooner or later I expect they'll decide to let it go and/or essentially ask for another apology before it's laid to rest. You need to decide whether you're willing to swallow any pride and move on without an apology from them for the sake of peaceful relations (while privately thinking what you like about them) or this is your hill to die on. That's genuinely up to you but just make sure that you're doing it consciously.