I thought your response came from the OP. I was wrong and shouldn’t have addressed you in the way I did.
To answer your question about shorting, there isn’t anything illegal about shorts in general but Melvin engaged in the practice of naked shorts (which does have a lot of illegal implications) and shorted more than 100% of company shares. It’s an unethical practice on its own but then Melvin took it further and manipulated the market further to drive the price down to get even more profit. Many have called for the SEC to investigate the situation Melvin created and to hold them accountable in any legal way possible.