How to cope with intensely hating somebody and knowing the truth when everybody else believes them?

[Plunker]

I have name changed for this because a few people in real life do know the situation and it would out me. I'm not sure relationships is exactly the right place for this, but I don't know where would be. My mother in law died last year and this is about her husband. She married him when given a few days left to live, for the wrong reason. He is a vile, controlling man and this has given him the power he so desperately wanted. My mother in law was also one of my best friends and I feel so, so guilty that I didn't make myself absolutely clear afterwards and make it so that she could prevent him from having the control he now has. She was terminally ill and I thought it was the last thing she needed to deal with. This situation has been used to his advantage because me/my husband/her family are good and nice people who would not go around slagging him off to people. He is that kind of person, so things go unanswered and people believe lies. My husband has been unable to grieve properly, because this man needs to constantly be the centre of attention and everything my husband has, or remembers, he has to involve himself in and make about himself. I will go into specifics in a bit - I don't have the time right now, but I NEED somewhere to tell the world what he's really done and who he really is. Even if it's anonymously, I need to say all of this publicly. Her friends believe him. Her memory has been dwarfed by him. I can't say anything, even though I desperately want to, because I don't know her friends well enough and all it would do is cause drama and make me sound crazy and bitter. He has already got I there and manipulated people. It would only make things more awkward for my husband and brother in law.

I know this is quite a self indulgent post and it's silly that I've posted with the intention of giving details later, but I'm struggling to cope with knowing that he still exists this morning and need to just say something. And I would definitely appreciate pointers on how to 'let go'. I don't want to feel this level of hate for the rest of my life. I think that I've successfully let go and come to terms with things, then somebody else brings him up or mentions something about him and I am hateful and anxious for another few weeks. I honestly find him so, so creepy. I wish I could turn back time and prevent her from marrying him in her drugged up state.

These feelings can't just be switched off, you know that, they're real and they hurt and they're all consuming at the moment. What you have to do is work through them and try to understand them.
Writing it all down in a dedicated journal will help you to offload some of the anger you're feeling. Write daily, keep a diary in which you confide all your thoughts it can be immensely therapeutic for this type of trauma.
And that is what it is. Its a trauma and you have to be kind to yourself as you seek to get some closure on all of this. If others don't see what you see then accept it. You can't make people see things differently but you know that I think.
What you can do is distance yourself from this person. Don't be available, don't offer to do anything, reject any overtures and find your own coping mechanisms in the process.