vendetta - A blood feud in which the family of a murdered person seeks vengeance on the murderer or the murderer's family

As the final act of his vendetta against Rugen, Inigo pulled out his sword and said, "Hello. My name is Inigo Montoya. You killed my father. Prepare to die."
senile - Used to describe behavior, mental states, or conditions associated with aging, especially when a person becomes forgetful or confused.

The elderly man showed symptoms of becoming senile, like forgetting where he put his keys, but with support, he managed to live independently for a while.