Yes, expulsion is pretty much the end of any hopes of becoming a doctor you may have had. Even undergrad schools, which are infinitely easier to get into than medical schools, are very wary of taking applicants that were expelled from their schools (either high school or another undergrad institution). In the graduate school world, even having an IA with no disciplinary action on your record is enough to sink your application at every school in the nation.
It's odd that you mentioned drinking/drugs and assault as the two hypothetical scenarios, and then proceeded to focus on drinking as reason why someone might get expelled. Let me guess: You got drunk, campus police busted your party, you got into a fight with campus police or another student (or both), after which your school expelled you, and now you're hoping that you can just list one of the reasons (ie the least worst one) as the reason why your school expelled you on any future med school applications.
Am I on the right track with this?