Two reasons you should not short Bitcoin.

1) Actual legitimate Bitcoin payments are increasing and real.

https://blog.bitpay.com/bitpay-growth-2017/

2) Even if this whole thing does collapse, never short something like Bitcoin unless you have unlimited cash to support such a short. Even if you're right, can you continue to sustain your short if the peak is $100,000 a coin before the collapse? (And the new bottom is $10,000, still higher than right now)

If the price goes there, can you afford the short?