Temper tantrums are not a symtom of mental retardation. Of course, any human being can have a temper tantrum, but it's not something that I've seen very often in a 29-year career working with individuals with disabilities.
Sounds like your neighbor has at least one other diagnosis, too. The DSM-IV is full of diagnoses - psychosis, neurosis, behavioral problems, etc. Could be anything, or nothing out of the ordinary, but it sounds as if you've seen this behavior more than once or twice. (I had a neighbor in Baltimore when I was kid. If his mom wasn't home when he got there after school he would strip naked and pee on the front door. If you put your hand on 'his' fence, he'd whack it with a croquet mallet. I now know that he was not retarded, just seriously disturbed.)
The typical person with mental retardation, including those diagnosed with Down's Syndrome, make excellent employees when trained on the actual job by someone who knows how to structure the work and teach the coworkers and bosses the ropes. For instance, a good entry-level job can be bagging groceries and taking them to the car - but the boss can't decide to put the person on the register because somebody didn't show up that day. Isn't going to work. You would think that this would be obvious? Ha. There's no underestimating the stupidity of so-called normal people.
I now return you to the usual good advice on self-defense issues.
John