In "To Kill a Mockingbird" how is Arthur "Boo" Radley a threat to Maycomb?I was given the prompt of arguing that Boo Radley is a threat to Maycomb,...

That's a tricky question, and I don't doubt that you are having problems coming up with support for it.  The thing is, Arthur "Boo" Radley really ISN'T a threat to Maycomb at all.  Even back in his young days, when he was brought before the judge for unruly behavior, he wasn't a threat.  He and his buds were just being stupid teenagers, and no real damage was done.  From that point on, Boo just stayed in his house, and didn't bother anyone at all.  The only mildly threatening thing that he might have done was stab his father in the leg with scissors--if that even happened, from what we learn of his father, it's hard not to blame Boo for being frustrated with him.  And that act doesn't make Boo a threat to Maycomb at all; it was one incident, probably provoked by his father, if it even happened at all.

There is only one potential way that Boo is a threat to Maycomb, and that is in relation to their fears and prejudices.  If the townspeople knew what a kind, loving, friendly and lonely guy that Boo was, and that he actually wasn't dangerous like all of the rumors claimed he was, that would threaten their preconceived notions and judgments.  Their ideas of people would be threatened; their stereotypes in regards to other people and their appearances would be shattered.  People like their stereotypes, they like their assumptions about people, they like their boogeymen, and they like to feel superior to other people.  Boo Radey threatens all of those things.  He isn't what they assume he is, so, they would have to admit that they were wrong, and that they aren't any better than him, a social recluse that is a bit strange.  It would threaten their world-view, and their sense of security and esteem that they gain from their assumptions and prejudices.  I hope that all of that made sense, and helps you out a bit; good luck!