At some point, everyone has the experience of living near a creepy or suspicious neighbor. Sometimes it's nothing you can specifically point to; it's the neighbor's manner, the hours they keep, or the emanating smells. Other times, it can be more concrete: troubling noises, anger, or actual threats against you or your family.
What should you do if you're scared of your neighbor?
Recall your high school English class when you read Harper Lee's To Kill a Mockingbird. "Boo Radley" was a frightening character for much of the novel, a loner who was feared by the neighborhood. Boo was a source of terror largely because of his strange mannerisms and the mystery surrounding his past. At the end of the novel, however, it is revealed that Boo is a supremely decent guy, playing a key role in aiding the protagonists.
Thus before you overreact to what you perceive as your neighbor's oddness, consider that this person might simply not present well, or might come from an unfamiliar background or culture. Perhaps the neighbor has mental health issues, but is wholly nonviolent. Do not be quick to judge, especially if your neighbor's behavior isn't directly harming you.
If the neighbor appears rational and approachable, and there's a particular issue that you'd like to see addressed (other than "stop acting weird") mediation might be an option before you try more extreme measures.
Of course, you will want to keep your eyes open for any behavior that crosses the line and involves actual threatening or inappropriate action.
If you live in an apartment building or in a community governed by a neighborhood or homeowners' association (HOA), that might be your best first stop. For example, imagine a situation where there seem to be an unsafe number of animals in the neighbor's apartment causing a ruckus, or you regularly hear screaming inside a neighbor's home.
All homeowners (or condo or townhouse owners) in an HOA must abide by detailed community rules. These might, for instance, cover what pets are allowed, and include noise limits. It's entirely appropriate for the HOA management or neighborhood association to intervene, addressing the neighbor's conduct while leaving you out of it.
In a situation where you have reasonable, immediate fear of your neighbor, particularly if the person makes physical threats or engages in any actual violence or property damage, gather any evidence you have of the issues and call the police. A knock on the door from law enforcement might be the best means of sending your neighbor the message to stay away from you and your family.
Keep in mind, however, that once the police respond to your call, they might have you fill out a report. Your neighbor is then likely to learn that you were the person who complained. The more independent evidence you can provide, such as photos of dangerous activity by your neighbor, the less personal involvement you might need to have in the case.
Going to court could be an option, for example with a nuisance claim. However, it seems unlikely that a private lawsuit between you and your neighbor would address the issues described here.
Courts tend to be most effective for money damages. Your neighbor might not have harmed you in a quantifiable, financial way, but rather in an emotional way, by creating fear.
What's ahead for you and your neighbor? The long-term outlook and your corresponding strategy will depend on whether you and your neighbor are owners or renters.
If your neighbor owns the property, and you're merely a renter, that reality might factor into your decision about renewing your lease. If you're in a multi-unit building, particularly in a large city, you might be able to consider creative alternatives with your management company. It might be willing to let you switch apartments within the same building. You might be able to convince the company to allow you to pay a little extra to move apartments even before your lease expires. This could allow both you and your neighbor to live in peace, but farther away from one another.
If you are a homeowner, the matter could be more complicated. In the worst case, you might consider selling your home. Realize, however, that depending on your state's laws, you might need to disclose any material problems to prospective purchasers. A difficult neighbor could unfortunately reduce the value of your property.