What can I do about a mentally unstable neighbor?

Question Details: I have a problem and I am not sure what the best course of action is. I have a neighbor who lives next to my 91 year old grandfather. This neighbor is very "odd". He has been aggressive yelling at my grandfather, he steals from his garage which has been witnessed by other neighbors, although he's never caught and he pees on his house. However, the most concerning is that he starts fires in the small space between the 2 houses. I'm worried that he is going to set the house on fire. The fires I have caught on video but the police say they can't do anything about it unless he is caught in the act. I had cameras installed for my grandpa and I have caught him flipping off the cameras, peeing in the yard and lighting fires. I don't know what to do. The police won't do anything and my poor grandfather is so worried that he won't go out of his house if he's home. The neighbor has substance abuse issues and has been arrested and treated for mental health issues in the past. I don't want a war with this guy, I just want him to leave us alone.

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