4 Answers

Ask the police or your local authority.

If the neighbours are unaware of this persons behaviour, it might be a good idea to warn them, but do it discreetly, you don't want this madman coming after you.

It would be better to talk to that neighbor directly, as anything in print can come back to haunt you...  Or you can search for a website that shows location of people with felonies, and give that address to the neighbor to "discover about him", then you didn't do anything that wasn't already public!  Here, in US, criminals seem to have more rights than law abiding citizens.. 

You can not slander people, so I would be very causious as to how you go about this, if your renting I would suggest you just move, it only takes one person to tell your neighbour and then the danger will esculate..