my granfather was accusing my grandmother about messing around with someone and i heard the argument so i kept saying she could never do that only because she is always with him so as he was saying im lying, he lift his hand and was about to hit her, pulled her away and he punched me in my mouth and a blur after that. all i know is that he said its none of my buisness and hours later he doesnt remember what happened. what do you suppose we do. he already said he was going to kill her and her never seems to sleep because hes always asking her questions about money and he so called boyfriend he made up.
This kind of paranoia and delusions is fairly common with dementia. Your grandfather can't help it and you must not think he is a monster or wicked. The disease is monstrous, that is for sure!
While I have great sympathy for Grandfather, his behavior is putting you and his wife in danger. It is amazing how strong some of these frail little ol' guys can be in a fit of emotion. I am glad you are taking this seriously. Talk to his doctor. Talk to Adult Protective Services. Be sure to mention the threat to kill her.
In the meanwhile, if possible try to calm Grandfather down during his outbursts by acknowledging his feelings. "Oh Grampa, how awful you must feel. It must be painful and very sad to think that after all these years Gramma is unfaithful to you. But here is what I know. Gramma loves you very much. She is here with you all the time. I am so very sorry that you saw something that makes you think she is running around but I know that she isn't. We both love you and want you to be happy." I'm offering no guaranty that this will work, but it is worth a shot.