I think it gets harder to 'gun' people as you go along. Eventually you are judged by what you can actually do and how well you can do it, not by how you compare in terms of potential to other students.
If anything, the people who work well with others tend to do better -- publish more, do better work, etc. There are definitely people out there who will steal your research ideas, but that's why you don't share them with just anyone and you work hard and get it done asap. Medicine and research at the highest levels are very much team sports. Notice there aren't many single author papers in JAMA or NEJM! No, gotta be able to play well with others if you're going to truly get ahead. Maybe you could blackball people quietly behind the scenes, especially people who might be competition for some position you want, but you would run a huge risk of making yourself look worse in the process.
Maybe it's because past a certain point, it ceases to be a zero sum game. It's not like my successes have any impact on my colleagues ability to succeed and vice versa.
