In the classic 1976 film Taxi Driver, schizophrenic Vietnam veteran Travis Bickle (the eponymous taxi driver) eventually develops a plan to go on a killing spree (and does, towards the end of the film.)
At a point in the middle of the movie, Bickle confides to another character that he has "some bad ideas in [his] head," and is the last point in the film where he could be "saved" mentally, and not start the climactic shootout at the end of the film.
Now, I by no means idealize Bickle; he's obviously crazy.
But.
I have some bad ideas in my head.
Since at least middle school (some five years ago or more) I've had fantasies involving gun violence in school, sometimes committed by me, and sometimes not. When it happens, I push it out of mind and forget about it.
But now that I've seen Taxi Driver and I've seen what can result of that...
...I'm scared s**tless. Really, truly, and deeply afraid of what could happen should I lose control of myself.
Which, hopefully, will never happen. I personally doubt it will; I think that I'm personally incapable of doing it; I care far too much about far too many people there to do anything of the sort.
But the fact that those fantasies exist scares the crap out of me.