My wife and I were discussing that the other day.
In a martial law type situation you would be wise to realize that a sickening number of your neighbors would turn you in to DHS in a heartbeat to get extra favors like ration coupons, etc.
They would also do it to settle petty grievances.
Yes, some of your neighbors will be your enemy when the SHTF. Guaranteed.
Or just out of general malice. Think about home owners' associations and the malicious busybodies in those who go around turning others in because of flagpoles, fences, etc.
In my neighborhood there are a couple of a**hole busybodies (retired) who go walk around a couple of times every week looking for things to turn people in about. Captured one of them on my surveilance camera looking in my garage. I didn't say anything, but now I know whose lawn on which to dispose of any waste oil that I might accumulate while working on my car.