Defending Mom

My father came to visit yesterday as he does pretty much every Monday and as usual by the time he leaves I'm in the worst mood. Mom passed away 2.8 years ago and he still complains about her, which is very upsetting to me. I've mentioned several times to him that they both could have been nicer to each other and I really don't want to hear anything negative about mom. But once a week he will slide some kind of derogatory comment into the conversation and yesterday was no exception. I won't share with you the response I gave this time because it was fairly sharp and I do regret what I said. For years it felt like they would put me in the middle of their arguments, like a referee or interrupter. I wouldn't take sides just listen and try to explain to the other parent what the first one was trying to say or meant. Now that she's gone it seems like I'm speaking out for her because A) she died and B) she rarely defended herself in the first place. It hurts me deeply and when he's in that kind of mood I would almost rather him stay away. What should I do? I've told him several times they both had faults, it hurts me to hear bad things about Mom and I don't want to get into an argument about it. He thinks by making short flippant comments that he isn't actually "saying bad things" about her but it's the same thing because I know where his mind is and where it's headed. Hell, I still struggle with my emotions if I spend more than an hour in their home. Everything I see there is exactly where she left it, like she just stepped out or ran to the store. I tried to get him to redecorate to help him move forward a bit but he won't. I don't know what more I can do or say that will help him move forward. Or better yet forgive the past. That's it, maybe I need to talke to him about forgiving her for all the things he's still upset about.