The only real choices you have are to set up surveillance on him. That doesn't sound very realistic either. Your neighbor can do whatever he wishes to your cats and unless he has claimed to kill them by writing it down or being recorded, he can deny it and many police will just ignore it. Most LE don't give a crap about the animals unless it's a cow. If you move the cats to your G-ma's they will be very confused and some WILL run off! They would have to be inside for a time to understand it's THEIR new home.
I hope you can endeavor to get this creep for being so malicious to animals.
I hope you can endeavor to get this creep for being so malicious to animals.