Don't know what school of meditation you were teaching but in Buddhist meditation there is nothing to be afraid of. It is perfectly safe. Sure you do see "bad things" flash across your mind, but you just ignore them and let them go, that is the whole point of meditation. The "bad stuff" actually comes from you. By just watching them and not responding to them at all you're let them go. Clearing your mind. The key is to do nothing, and certainly never react with fear or you would have made them real to you. All these "bad stuff" are just pure illusion.