I had a d* neighbor, back in the day, who tried to kill the tree on our side of the fence line. Not sure what his issue was, but this tree was a mammoth of a thing and very much alive. He had people come in once and trim HIS side. Then they started going a little too far and gored the heart of the thing, per his instructions. They were ordered to stop else face legal action.
A few years later, he approached me yet again to trim the tree. This time, he (initially) politely asked if we'd get it trimmed. I looked into it, found a crew who could do the work who were bonded/insured and who'd do it for a reasonable price. Within a day or two of the appointment, he had notice of, he mentioned he didn't any of the vehicles to venture onto his property. 'Excuse me?' In order for the crew to get to at least half the tree, they'd have to venture there. Otherwise, the cost would increase exponentially and require special gear. HE was the one who asked we trim it. Seriously?
Ultimately, I had the tree trimmed as I wanted (just a tidy job taking out a few smaller storm-broken limbs), without doing anything on his side. Then I had a certified agent from the state's arborists association survey the health of the tree. With their report, I contacted an attorney and had him inform my neighbor of the legal state of the tree and his inability to touch it, else he'd be fined a minimum of $10k. If he inflicted harm that resulted in the death of the tree, or cut it down, criminal charges as well as additional costs would be levied. That a**hole never called me again.
Now, having said that. I have had both good and bad neighbors. And we have certainly tried our best to be respectful, helpful ones. Normally, I wouldn't go to the point of legal action. But he was unreasonable and demanding and gave me no other choice but to meet him at his level.