i'm a first year at wash u so hopefully i can answer some of your questions/ease some of your concerns. first of all, as for the competitiveness/grading system, yeah, there are a ton of really smart people here. but there are also a lot of non-trads in our class, people who have never had biochem or physio, who have only done the minimum science prereqs and the like. and first year is pass-fail. in the hopes that my father won't be visiting this site, i'll admit that i never go to lecture. ever. and, except for the week before an exam, i don't really study either. and i'm smart, but i'm not that smart, and i'm doing fine. everyone wants you to do well. so don't be concerned about being in above your head, because everyone is concerned about that and everyone does fine.
next, the safety concern. there have been some robberies close to campus this year, which definitely isn't cool. but i don't think st. louis is more dangerous than any other city. the med school itself is located in a nice area, but if you get about a mile away it gets a little sketchy. as long as you live nearby and don't go wandering away from campus at night, you'll be fine. it's no different than living in any other city.
and then the whole matching question. i don't have dreams of matching to mass general or someplace huge and prestigious like that, so i haven't done a ton of research into stuff like that. what i do know is that wash u is an incredibly well-known institution and people like wash u students. they know that everyone here is smart and that even really talented students might not end up in the top half of the class. i don't think being average here would be detrimental to getting into a good residency. i think the name helps.
as for your last question, i don't think you should choose your schools based on where you think you'll end up in the class rankings. you should go to the place where you think you'll get the best education, where the teaching style fits your learning style, where the clinical opportunities are the most extensive, etc. in the end, i really think it's important to take the focus off of the numbers and onto the more intangible parts of your education. if you go somewhere where you're happy and you fit, no matter how prestigious the institution or how much smarter (or dumber) than you your classmates are, you will end up with a good residency and a good career.
pm me if you ever want to talk about wash u. that goes for anyone. and good luck deciding!