What would kill Bitcoin? Other than banning it, what is the biggest risk to the price of Bitcoin? What circumstances may kill Bitcoin or bring it down to less than $10?
No one can determine the effect certain threats will cause, but some "popular" threats are:


Extreme government regulation.
Bitcoin losing to a competitor coin.
Technical attacks, including attacks on protocol (51% attack) or attacks on cryptography.
Intentional market manipulation to crash the price.


Some factors that can drive these events to be more likely to occur or enable them are:


Unresolved technical issues, such as blockchain bloat, or transaction limits within blocks.
End of or diminishing block rewards causing people to abandon mining.
Increased centralization for whatever reason (advanced technology, zero power costs, etc.)