In such a case I would just assume you are stupid or something and keep on my business. If you don't have capacity to ignore that kind of noise you probably need treatment. Threats, however, are a different story.

You shouldn't care much if some random people express their opinion which is you're a moron (unless you meet such people often - then it might be a good idea to actually make sure there is nothing wrong with you or you are not hanging out in a wrong place) - ignoring insults usually causes better consequences than reacting to them. At the same time you usually should be worried if somebody threatens to kill you, ignoring threats can often get you in a real trouble.