The same BlackRock that's just made a killing short selling shares in the failed UK public sector contractor Carillion?
Short-sold stocks are 'borrowed' from an investor who has less market information than the trader.
The trader deliberately hides that information so as to leave the owner holding the bag when the price drops, whilst pocketing the difference as profit.
Doesn't seem ethical to me. Legal, yes.
[0] https://www.cnbc.com/2017/12/14/jim-chanos-we-think-tesla-is...