1) You are forgetting time. If this information leads directly to your probability of being murdered, that would make it about 1/18000 PER YEAR. If you live the first year 17999/18000, there is more probability of your demise in the second year. This might lead to the final answer:
1/18000 + (17999/18000)*(1/17000) + [(17999/18000)^2]*(1/17000) + [(17999/18000)^4]*(1/17000) + ...
If you add these up for 101 years, this suggests about a 0.6% chance over a 101 year lifespan.
However, there is some chance you would die of some dread disease or auto accident or something else, so this 0.6% over a lifetime is a gross over-estimate. Still more, this value assumes the probability stays the same over the entire span of 101 years. This is an assumption that is very unlikely to be observed.
2) Now the real answer. Your 1/18000 is a population average. You absolutely MUST NOT apply population averages to individuals. It is irresponsible! Your chance of being a homocide victim is FAR MORE related to your behavior than to your city. Your neighborhood is more important than your city, too, but your neighborhood may influence your behavior. There are some murders that are very "random". but not very many.