I don't think there is such a thing as a bad trip, You can have an experience, which seems subjectively negative, but it's not a bad trip. Old american Indian Shamans, who are very orientated in realms of human consciousness, say that, sometimes the experience takes you into the underworld, and sometimes it takes you to higher realms. it depends where you are at.
From my personal experiences, I can tell you, that the so called "bad" trips are just as good as the good ones. During the psychedelic experience, you see yourself as you truly are, so if you're a bastard, you are left to deal with that. If you're not honest with yourself, and always run from yourself, you can get caught out during the psychedelic experience. You are simply forced to look at yourself. And work shit out. And if you're ego is too big, and you don't want to face your shit, negative experience occurs. ANd it will keep occurring. Until you work your shit out. So really, no such thing as a bad trip. Just bad parts of your own self.
It's just about being honest with yourself.
So to put it into a sentence, during "negative" experience, you are identifying parts of yourself that no longer serve you. During the positive experience, you are creating new parts of yourself which will serve you for better than the old ones.
Love and Light :D