Some drugs seem to lend themselves more to scary trips than others. None of the datura trip reports I have read seem pleasant, most people seem to never touch it a second time. Heard quite mixed things about Amanita's as well. I think anyone that does datura (and has done even a little research on it) is expecting an unpleasant trip, and probably wants to do it still out of some sort of curiosity.
As for controlling trip with intent, I think it would be very difficult. You could control it with setting though, haunted house, very busy places, important social settings, ect. You could force yourself to have a bad time through that. There used to be a vice series where a guy did acid in a wide variety of settings, seemed pretty miserable. I can't see what you would gain from that though.